Prochains séminaires
| Matthieu Delbecq (LPENS (Paris)) | Détails Fermer |
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Universal fluctuations of the induced superconducting gap in an elemental nanowire le mardi 26 septembre 2023 à 14:00 |
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Résumé : Proximity induced superconductivity in a normal conductor is a rich field of experimental and theoretical investigations in many systems. In the last decade, it has been particularly at the heart of the quest for realizing topological modes in hybrid superconductor-nanowire nanodevices. Yet surprisingly, it turns out that there was a clear lack of investigations in simple systems. We therefore took on to investigate an elemental nanowire in the 1D limit (an ultra-clean carbon nanotube) coupled to a superconducting lead. We observe for the first time a long standing prediction of random matrix theory (RMT), dating back to 2001, that mesoscopic fluctuations of the mini-gap in a conductor follow a universal distribution (1). The statistical distribution of the mini-gap recorded over 60 consecutive charge states in our device shows a universal behavior with a transition when time reversal symmetry is broken, as predicted by RMT. Interestingly, mesoscopic fluctuations of the minigap were precisely predicted to lead to ubiquitous nontopological edge states clustering towards zero energy. We do indeed observe ubiquitous and robust zero bias conductance peaks in our device that cannot host topological modes by design, as expected by RMT. The RMT predictions that we confirm are very general and must be present in any system showing disorder, even if it is weak. It therefore unambiguously calls for alternative probes than transport measurement to identify Majorana modes in 1D systems. Microwave photons in a cavity are a promising powerful platform (2) that I will discuss. Ref.: (1) L. C. Contamin et al., Nature Communications 13, 6188 (2022). (2) L. C. Contamin et al., Npj Quantum Inf. 7, 171 (2021). Liens : |
| Andrea Tononi (Université Paris-Saclay, CNRS, LPTMS) | Détails Fermer |
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Self-bound fermionic mixtures in low dimensions le vendredi 22 septembre 2023 à 11:00 |
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Résumé : Self-binding in Bose-Bose mixtures has received lots of theoretical [1] and experimental [2] attention in the recent years, and a few studies also discussed Bose-Fermi droplets [3]. Fermi-Fermi mixtures with zero-range interspecies attraction, however, are not expected to display self-bound states, since the fermions of one species should overcome a strong Pauli pressure to bind the fermions of the other. This repulsion is, in fact, the fundamental mechanism that provides stability of Fermi mixtures along the BCS-BEC crossover, in which the dimers repel and do not form larger clusters [4]. In our work [5], we find that a 1D Fermi-Fermi mixture with sufficiently large mass imbalance can form a self-bound state in the thermodynamic limit. This result elaborates our previous few-body analyses [6], and is based on a mean-field theory in which the heavy fermions are described within the Thomas-Fermi approximation, which is exact in the limit of large mass ratios. We are also extending our theory towards the 2D case, which is complicated by the same scaling with length of the kinetic energy and of the interaction energy of the system [7]. Our work sets the basis for understanding liquid-like states in fermionic gases. References
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| Christian Schönenberger (University of Basel) | Détails Fermer |
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Search for the Fractional Josephson Effect in Topological and Nontopological Materials le mardi 19 septembre 2023 à 14:00 |
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Résumé : I will report on an extensive search of the AC Josephson effect of superconducting junctions and weak links obtained from various materials in a low-ohmic environment allowing for DC bias. The materials are two-dimensional graphene, Al proximitized InAs quantum wells, the Dirac semimetal cadmium arsenide, the Weyl semimetal tungsten telluride WTe2, the 3D topological insulator HgTe, InAs nanowires and carbon nanotubes, as well as conventional Al-based reference Josephson junctions. We have studied AC Josephson emission, Shapiro steps, current-phase relations and Fraunhofer patterns to address the current distribution. We can observe missing Shapiro steps, but there is no AC Josepshon signal appearing concurrently at frequency eV/h, as expected for topological junctions. However, we find strong higher order Josephson terms that go with frequency as 2neV/h with n=1,2,3... These terms stem from simultaneous inelastic tunneling of n Cooper-pairs. The relation to the skewness of the CPR will be addressed. I would also like to stress that we observe missing Shapiro steps even in standard conventional Al-Al-oxide-Al Josephson junctions. Hence, missing odd Shapiro steps cannot serve as a signature for topological superconductivity, although this signature has been used multiple of time for the in the concurrent literature.
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| Louis Garbe | Détails Fermer |
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KPZ fluctuations and bosonic skin effect in the ASIP model le mardi 12 septembre 2023 à 11:00 |
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| Mikhail Feigelman (LPMMC, Grenoble) | Détails Fermer |
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Theory of the 1st order Superconductor-Insulator transition le jeudi 13 juillet 2023 à 11:00 |
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Résumé : Recent experimental studies of transport in strongly disordered Indium Oxide films revealed an unusual first-order phase transition between superconducting and insulating state, with the jump of kinetic inductance of the system at the transition. In the present work we propose interpretation of this transition as the transition between superconducting state built on top of Cooper pairs localized due to disorder present in the system, and a Coulomb-glass type insulator with developed Coulomb gap. To describe this transition analytically, we present a theoretical model formulated in terms of Anderson pseudospins. Utilizing mean-field Parisi replica-symmetry breaking scheme, we demonstrate the existence of a region in the parameter space where both phases are locally stable w.r.t. small fluctuations, which is the necessary condition for the first order phase transition. Finally, we provide estimates on the free energy of both phases and the position of the transition itself. The talk will start from an extensive exposition of the SIT subject, which has a long history. The seminar can also via followed via zoom https://univ-grenoble-alpes-fr.zoom.us/j/99884767935?pwd=dmpSVGVFZ2I5WkQyNEFLN2t1dVhEQT09 (Meeting ID: 998 8476 7935, Passcode: 630179) Liens : |
| Tomas Ramos (Institute of Fundamental Physics Madrid) | Détails Fermer |
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Characterization of QND Measurements and Topological Amplification in Superconducting Circuits le mardi 11 juillet 2023 à 14:00 |
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| Kirill Shtengel (UC Riverside) | Détails Fermer |
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Experimental signatures of non-Abelian braiding in quantum Hall systems at nu=5/2 and 7/2 le vendredi 30 juin 2023 à 11:00 |
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| Robert Whitney (LPMMC) | Détails Fermer |
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LPMMC discussion: Pursuing an Academic Career after a PhD in Theoretical Physics le mercredi 28 juin 2023 à 11:00 |
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Résumé : A round-table discussion about academic careers in theoretical physics, led by members of the LPMMC staff. It is intended for PhDs/Postdocs of LPMMC, but theoretical physics PhDs/postdocs from other labs are welcome. The discussion will start with a presentation on the practical aspects: when to apply for CNRS and University jobs (France and abroad), what juries are looking for, etc. It will then move on to discuss how you judge if a academic career is suitable for you, and how to maximize your chances. Liens :Robert WhitneyLPMMC |
| Antimo Marrazzo (Trieste) | Détails Fermer |
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Finding 2D topological insulators with computers: crystals, disorder and temperature le vendredi 23 juin 2023 à 11:00 |
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Résumé : Almost two decades after the theoretical formulation of the quantum spin Hall insulating phase (QSHI), the number of real two-dimensional (2D) QSHI materials which have been confirmed experimentally is relatively limited, often exhibiting poor performances. Indeed, the intrinsic robustness advocated by topological protection can still suffer by the presence of defects or temperature effects, as most real QSHI can be easily perturbed into a metallic or trivial state. In addition, theoretical predictions of QSHIs can be very sensitive to the accuracy of the electronic-structure methods employed. Hence, fundamental research and potential technological applications of QSHIs are hindered both by the rarity of high-performance topological materials and by the lack of predictive modelling in complex settings, ranging from strong correlations to disorder, to finite temperature. In this talk, I will first present some achievements in the discovery and design of novel QSHI with first-principles simulations. In particular, I will discuss our discovery of jacutingaite, a naturally-occurring dual topological insulator made by potentially-exfoliable monolayers. 2D jacutingaite realizes graphene’s Kane-Mele model with a large band gap and a nice interplay between spin-orbit coupling, crystal-symmetry breaking, and dielectric response. Beyond materials discovery, I will provide an example of materials design and discuss our prediction of robust ferroelectric QSHI states in van-der-Waals heterobilayers made by two non-topological monolayers.  Finally, I will present more recent efforts towards ab-initio modelling of disorder and temperature in QSHIs. In particular, I will introduce single-point and space-resolved frameworks to calculate the Z2 topological invariant and other geometrical quantities for non-crystalline systems. These latest efforts are being released in a dedicated software package, SPInv, which is designed to work both with model Hamiltonians and first-principles simulations, operating in the Wannier function software ecosystem. Liens : |
| Olivier Gauthé (CMTC, EPFL ) | Détails Fermer |
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Tensor network approach to strongly correlated systems at finite temperature le mercredi 21 juin 2023 à 11:00 |
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Résumé : Over the last decades, tensor network methods have emerged as the one of the most powerful numerical methods for tackling the many-body problem in quantum physics. In this talk, we will review the core principles of tensor network and their applications in condensed matter physics. We will focus on two dimensional systems and discuss simulating strongly correlated systems at thermal equilibrium. In such systems, implementing efficient imaginary time evolution and tensor network contraction proves challenging due to the requirement of very large bond dimensions to maintain physical relevance at low temperatures. In addition, it is essential to preserve good numerical accuracy to avoid unphysical symmetry breakings that may hide important physics. The solution for both issues is to incorporate non-abelian symmetry directly at the tensor level, which yields substantial improvements in performance and precision. To illustrate the potential of this approach, we will present results on the spin-1/2 Heisenberg model on the square lattice with nearest-neighbor coupling J1 and next-nearest coupling J2 (J1-J2 model) at finite temperature [1]. We will introduce several ways to detect the thermal phase transition occurring in this model and generalize them to related models. Reference[1] O. Gauthé & F. Mila, PRL, 128, 227202 (2022)Liens :Olivier GauthéCMTC, EPFL |
| Anne Anthore (C2N) | Détails Fermer |
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Observation of a Kondo impurity state and universal screening using a charge pseudospin le mardi 20 juin 2023 à 14:00 |
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Résumé : The Kondo effect, deriving from a local magnetic impurity mediating electron-electron interactions, constitutes a flourishing basis for understanding a variety of intricate many-body problems. Its experimental implementation in tunable circuits has made possible advances through well-controlled investigations. However, these have mostly concerned transport properties, whereas thermodynamic observations - notably the fundamental measurement of the spin of the Kondo impurity - remain elusive in test-bed circuits. In this talk, I will present how we directly observe the state of the impurity and its progressive screening with a novel combination of a "charge" Kondo circuit and a charge sensor. We establish the universal renormalization flow from a single free spin to a screened singlet, the associated reduction in the magnetization, and the relationship between scaling Kondo temperature and microscopic parameters. Liens : |
| Debanjan Chowdhury (Cornell University) | Détails Fermer |
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Useful bounds on superconducting Tc le vendredi 16 juin 2023 à 11:00 |
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Résumé : Superconductivity in the limit of a vanishing bandwidth in isolated bands is a classic example of a non-perturbative problem, where BCS theory does not apply. What sets the superconducting phase stiffness, and relatedly the transition temperature, in this limit is of both fundamental and practical interest. This question has become especially relevant with the discovery of superconductivity in moiré materials. I will begin by examining critically the relevance of various proposed bounds on the superconducting transition temperature and propose a non-perturbative upper bound on the integrated optical spectral weight for partially filled electronic flat bands with generic density-density interactions. I will also present numerically exact results for the interplay between superconductivity and various competing orders in models of interacting flat-bands. Liens : |
| Maria Spies (CNR NANO (Istituto Nanoscienze)) | Détails Fermer |
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Quasiparticle-based and Cooper pair-based superconducting diodes le mardi 13 juin 2023 à 14:00 |
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Résumé : Diodes are key elements for electronics, optics, and detection. Their evolution towards low dissipation electronics has lead to the hybridization with superconductors (S) and the realization of non-reciprocal transport of both quasiparticles and Cooper pairs. That occurs when both spatial inversion and time-reversal symmetries are broken. Here, we review both effects comparing their efficiencies and basic principles. The quasi-particle diode is a superconducting tunnel junction with zero conductance in only one direction. The directionselective propagation of the charge has been obtained through the broken electron-hole symmetry induced by the spin selection of a ferromagnetic tunnel barrier made of a EuS thin film separating a superconducting Al and a normal metal Cu layer. It achieves a large rectification of up to 40%. On the other hand, supercurrent diodes made with hybrid S/spinorbit/ S Josephson Junctions or with two-dimensional Rashba superconductors have been demonstrated to show zero resistance in only one direction. We describe the equation of the supercurrent diode effect in a generic formalism that may inspire novel devices based on helical magnetism induced in conventional superconductors. lien zoom: https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09 Liens :Maria SpiesCNR NANO (Istituto Nanoscienze) |
| Colloquium Steven White (University of California, Irvine) | Détails Fermer |
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Do the single band Hubbard models describe superconductivity in the cuprates? le vendredi 09 juin 2023 à 11:00 |
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Résumé : Since the beginnings of high temperature superconductivity in the cuprates, a key theoretical question has been whether the single band Hubbard model and its cousin, the t-J model, describe the superconductivity at least qualitatively. While initially this seemed like a simple question, the physics of both the cuprates and the models is now known to be much more complicated. A key complication is the presence of spontaneously formed striped arrangements of holes, which have been argued (by different people!) to either enhance or to suppress superconductivity. The models are very challenging to simulate, but even harder to treat analytically. In this talk I will present new simulation results from both DMRG and quantum Monte Carlo which are closing in on the ground state phase diagrams of these iconic models. Liens : |
| Matteo Votto (LPMMC) | Détails Fermer |
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A Walsh functions toolbox for hamiltonian and gate engineering in dipolar quantum systems le mercredi 07 juin 2023 à 11:00 |
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Résumé : External driving fields acting on qubits have been proven a useful tool in quantum simulation, both to improve coherence and to effectively engineer hamiltonian evolutions not accessible with the underlying quantum hardware. We propose a protocol to engineer 1) arbitrary two-body hamiltonians and 2) a universal set of gates with long-range connectivity, using a qubit system subject to a static dipolar Hamiltonian and local driving fields. Our approach relies on the circuit decomposition of the quantum dynamics, and the parametrization of the time dependence of the driving fields with Walsh functions, which allows to sequentially implement the various hamiltonian terms on specific subsystems. Furthermore, it can easily incorporate global pulse sequences to reduce decoherence, for which robustness criterion have been studied. Our proposal can be readily implemented in various quantum technology platforms, e.g. in trapped ions, or dipolar Rydberg atom arrays with qubits encoded in the hyperfine levels. We illustrate this toolbox with two examples: 1) a quantum simulation protocol for spin models and 2) the realization of surface and toric codes. Liens :LPMMC |
| MISSING (University of Pittsburgh (USA)) | Détails Fermer |
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Unconventional Josephson Effects in Hybrid Superconductor-Semiconductor Junctions le mardi 06 juin 2023 à 14:00 |
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Résumé : Hybrid superconductor-semiconductor nanowires came into focus due to their promise for realizing Majorana modes. They were also tried in quantum circuits such as transmon qubits and parametric amplifiers. The same factors that make them interesting for these applications are also associated with a host of interesting Josephson effects. The structures tend to be ballistic or quasi-ballistic, come with gate-voltage control, exhibit strong spin-orbit coupling, large g-factors and transparent interfaces. Because of this, we took to studying higher-order Josephson effects, Josephson phi0-junctions, the combination of these that leads to Josephson diode phenomena, as well as supercurrents through single one-dimensional subbands. We also keep and eye on effects related to Majorana and topology such as spin-polarized supercurrents and fractional Josephson effects, but so far have not found those. Liens : |
| Dmitri Khveschenko (University of North Carolina at Chapel Hill) | Détails Fermer |
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Dirac physics in graphene: scotch tape-induced relativity, chiral symmetry breaking, magnetic catalysis, analogue holographic correspondence, and more le vendredi 26 mai 2023 à 11:00 |
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Résumé : Graphene and other Dirac (semi)metals provide an experimentally accessible playground for a host of (pseudo) relativistic phenomena that would be hard (or impossible) to observe in nature. This discussion reviews some of those peculiar behaviors that can be probed in transport, tunneling, photoemission, and other measurements Liens : |
| Côme Fontaine (LPMMC) | Détails Fermer |
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A study of the scaling of the solutions to the Kardar-Parisi-Zhang equation in the tensionless limit le mercredi 17 mai 2023 à 11:00 |
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Résumé : The Kardar-Parisi-Zhang (KPZ) is a non-linear stochastic equation describing the growth of an interface. Although it has been thoroughly studied in one dimension, recent numerical simulations unveiled a new unexpected scaling regime in the tensionless limit, when the diffusion term vanishes. This new regime is characterized by a dynamical exponent z=1, in contrast of its value in the conventional KPZ regime (z=3/2) and Edward-Wilkinson (EW) regime (z=2) . In this work, we study this tensionless regime as an ultra-violet fixed point of the renormalisation group (RG). A numerical study of the flow to the infrared allows us to observe the scaling function associated to this fixed point when the effective coupling goes to infinity. In addition, we obtain the z=1 dynamical exponent using a large momentum expansion of the RG flow equation. Liens :LPMMC |
| François Parmentier (Université Paris-Saclay, CEA, CNRS, SPEC) | Détails Fermer |
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Heat equilibration of integer and fractional quantum Hall edge modes in graphene le mardi 16 mai 2023 à 14:00 |
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Résumé : The fractional quantum Hall effect is one of the most intriguing phenomena of condensed matter physics, where electronic interactions in a two-dimension electron gas subjected to a strong magnetic field lead to the emergence of highly exotic states with highly unusual properties. Among these, the existence of neutral edge modes, carrying only energy along the edges of the sample in a direction upstream to that of charge transport, has driven more than three decades of research. Their charge neutral nature has made them singularly challenging to probe, such that they were only first observed in 2010. Since then, many works have addressed the thermal transport properties of neutral modes, in particular whether they exchange energy with their neighboring counterpropagating charged edge modes. Significant progress was recently made on this topic, but an important question remained unanswered: can upstream neutral modes exchange energy and thermalize with integer¬-charged edge modes located up to several hundreds of nanometers away from them? This question is far from trivial, as it can profoundly change our understanding of the quantum Hall effect in terms of independent transport channels, and affect the realization of future experiments seeking to explore and exploit the remarkable properties of fractional quantum Hall states. We present heat transport measurements in quantum Hall states of graphene demonstrating that the integer channels can strongly equilibrate with the fractional ones, leading to markedly different regimes of quantized heat transport that depend on edge electrostatics. Our results allow for a better comprehension of the complex edge physics in the fractional quantum Hall regime. G. Le Breton, R. Delagrange, Y. Hong, M. Garg, K. Watanabe, T. Taniguchi, R. Ribeiro-Palau, P. Roulleau, P. Roche, and F. D. Parmentier, Phys. Rev. Lett. 129, 116803 (2022). Liens : |
| Jan Behrends | Détails Fermer |
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Coherent error threshold for surface codes from Majorana delocalization le vendredi 12 mai 2023 à 11:00 |
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Résumé : Statistical mechanics mappings provide key insights on quantum error correction. However, existing mappings assume incoherent noise, thus ignoring coherent errors due to, e.g., spurious gate rotations. We map the surface code with coherent errors, taken as X- or Z-rotations (replacing bit or phase flips), to a two-dimensional (2D) Ising model with complex couplings, and further to a 2D Majorana scattering network. Our mappings reveal both commonalities and qualitative differences in correcting coherent and incoherent errors. For both, the error-correcting phase maps, as we explicitly show by linking 2D networks to 1D fermions, to a Z2-nontrivial 2D insulator. However, beyond a critical rotation angle, instead of a Z2-trivial insulator as for incoherent errors, coherent errors map to a Majorana metal. This critical angle is the theoretically achievable storage threshold. We numerically find the angle 0.14Ï€. The corresponding bit-flip rate exceeds the known incoherent threshold. Liens : |
| Romuald le Fournis (LPMMC) | Détails Fermer |
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QED correction to the Abraham and Aharonov-Casher forces on Rydberg atoms le mercredi 10 mai 2023 à 11:00 |
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Résumé : Since many decades electromagnetic forces on globally neutral matter are controversial [1]. This controversy is imputed to the non unicity of the decomposition of stress-energy tensor into matter part and field part. More recently, experiment have been built to solve this controversy [2,3]. Thanks to these experiments, the controversy has been partially solved but there are still gray areas. On the theoretical side, different works have shown that electromagnetic forces could also get radiative corrections [4,5]. Rikken and V. Tiggelen measured the controversial Abraham force but couldnÂ’t exclude or support the existence of QED corrections to it [3]. In our work we show that Rydberg atoms are good candidates to observe QED corrections to the Abraham force. In a second time, we demonstrate the existence of an electromagnetic force on a Rydberg atom which have no classical equivalent to our knowledge and which is in reach of experiments. References
Liens :LPMMC |
| Yannick Seis (ENS Lyon) | Détails Fermer |
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Ground state cooling of an ultracoherent electromechanical system le mardi 09 mai 2023 à 14:00 |
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Résumé : Cavity electromechanics relies on parametric coupling between microwave and mechanical modes to manipulate the mechanical quantum state, and provide a coherent interface between different parts of hybrid quantum systems. High coherence of the mechanical mode is of key importance in such applications, in order to protect the quantum states it hosts from thermal decoherence. After an extended introduction to the field of optomechanics, we present the characterisation at millikelvin temperatures of a microwave electro-mechanical system featuring an ultra-coherent phononic-crystal membrane. The mechanical dissipation rate is measured down to 30 mK reaching a Q-factor of 1.5 billion, at 1.485 MHz mode frequency. Then we perform resolved sideband cooling on the mechanical mode, cooling it to its motional ground state nmin = 0.76 ± 0.16. We thus show the operation of an electromechanical system in the quantum regime, where its coherence time is estimated to be ~100 ms. We also show microwave-induced mechanical broadening up to 630 Hz, reaching manipulation speeds on the order of state-of-the-art superconducting qubits coherence times making our device a candidate for microwave quantum memories. Liens : |
| Atac Imamoglu (ETH - Zurich ) | Détails Fermer |
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Optical investigation of strong electronic correlations: magnetism in semiconductor moire materials le vendredi 05 mai 2023 à 11:00 |
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Résumé : Moire superlattices in two dimensional semiconductors have enabled the observation of a wealth of phenomena driven by strong electronic correlations, ranging from Mott-Wigner states to quantum anomalous Hall effect. In this talk, I will describe magnetic properties of van der Waals heterostructures forming a frustrated triangular lattice in the vicinity of Mott-insulator states of electrons. By directly measuring electronic magnetization through the strength of the polarization-selective attractive polaron resonances, we find that when the Mott state is electron doped, the system exhibits ferromagnetic correlations in agreement with Nagaoka model. Our observations, which are in agreement with DMRG calculations, provide a direct evidence for itinerant magnetism with a kinetic origin. Liens :Atac Imamoglu |
| Kirill Dobuvitskii (LPMMC) | Détails Fermer |
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Theory of quasiparticle-induced errors in Schrödinger cat qubits le mercredi 03 mai 2023 à 11:00 |
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Résumé : Understanding mechanisms of qubit decoherence is a crucial prerequisite for improving the qubit performance. The conventional theory of superconducting qubit decoherence by residual Bogoliubov quasiparticles [1,2] was constructed for qubits in equilibrium. However, the novel cat qubits of dissipative and Kerr types [3,4] are operated under non-equilibrium conditions. Namely, an external microwave drive is needed to stabilize the so-called "cat states", given by superpositions of coherent degenerate eigenstates of the effective stationary Lindbladian in the rotating frame. We quantify the effect of the quasiparticles on such driven-dissipative qubits by introducing additional dissipators which act on the density matrix of the cat qubit. We also account for the effect of the external drive on the quasiparticles along the lines of Ref. [5]. References
Liens :LPMMC |
| Alexandru Petrescu (Mines Paris) | Détails Fermer |
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Signatures of classical chaos in driven transmons le mardi 02 mai 2023 à 14:00 |
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Résumé : Transmons are ubiquitously used in superconducting quantum information processing architectures. Strong drives are required to realize fast high-fidelity gates and measurements, including parametrically activated processes. We show that even off-resonant drives, in regimes routinely used in experiments, can cause strong modifications to the structure of the transmon spectrum rendering a large part of it chaotic. Chaotic states, often neglected through the hypothesis that the anharmonicity is weak, strongly impact the lifetime of the computational states. Chaos-assisted quantum phase slips greatly enhance band dispersions. In the presence of a readout resonator, the onset of chaos correlates with high transmon-resonator entanglement, and an average resonator response centered on the bare resonator frequency. We define a photon number threshold to characterize the appearance of chaos-induced quantum demolition effects during strong-drive operations, such as dispersive qubit readout. More generally, chaos-induced phenomena such as the ones studied here are expected to be present in all circuits based on low-impedance Josephson junctions. Ref.: Joachim Cohen, Alexandru Petrescu, Ross Shillito, and Alexandre Blais, arXiv:2207.09361 ; Ross Shillito, Alexandru Petrescu, Joachim Cohen et al., Phys. Rev. Appl. 18, 034031 (2022) Liens : |
| Andrea Tononi Annulé | Détails Fermer |
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Self-bound fermionic mixtures in low dimensions le vendredi 28 avril 2023 à 11:00 |
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Annulé
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| Félix Helluin (LPMMC) | Détails Fermer |
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Blueshift corrections of a 1D exciton-polariton condensate le mercredi 26 avril 2023 à 11:30 |
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Résumé : Exciton-polariton are bosonic quasi-particles that arise from the strong coupling between light and matter. They are typically formed in a quantum well embedded in an optical microcavity, from the interaction between quantum well excitons and cavity photons. Under non-resonant pumping, it is shown that exciton-polariton can form a Bose-Einstein condensate (BEC) [1]. This out of equilibrium BEC is sustained in a stationary state by the competition between continuous laser driving and losses coming from the leakage of cavity photons. Recent studies focused on the coherence properties of such driven-dissipative condensates and established connections with the Kardar-Parisi-Zhang (KPZ) universality class [2]. In particular, it is now known that the variance of the phase of one-dimensional polariton condensates follows the KPZ scaling in space and in time [3]. Chemical potential corrections are extensively studied for equilibrium BECs [4]. However, a description of these corrections is still lacking in driven-dissipative condensates. In the defect-free KPZ phase [5] of a 1D polariton BEC, we investigate the blueshift stochastic fluctuations, analogue of beyond mean field chemical potential corrections. References
Liens :LPMMC |
| Alexis Coissard (Néel) | Détails Fermer |
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Imaging tunable quantum Hall broken-symmetry orders in graphene le mardi 25 avril 2023 à 14:00 |
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| Alexander Zyuzin (Aalto University) | Détails Fermer |
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Superconductivity in flat-band semimetals le vendredi 21 avril 2023 à 11:00 |
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Résumé : Flat-band materials may have relatively high superconducting transition temperature. Several systemswere proposed to host nearly flat bands such as, for example, multilayer graphene with rhombohedral stacking, twisted bilayer graphene, and semimetals with high pseudospin quasiparticles. However, the effect of the flat band on superconductivity can be twofold. Despite favoring Cooper pair formation, its nearly dispersionless nature can be a serious impediment to pair condensation. The Cooper pairs formation and their condensation may occur at different temperatures. Liens : |
| Loïc Herviou | Détails Fermer |
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Possible restoration of particle-hole symmetry in the 5/2 Quantized Hall State at small magnetic field le mercredi 05 avril 2023 à 11:00 |
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Résumé : The nature of the experimentally-measured fractional conductance plateau at filling 5/2 in Quantum Hall states remains an open question, with several candidate states presenting slightly different topological order. After a decade of debate, the theoretical consensus settled on the non-Abelian Antipfaffian state, Nonetheless, recent experimental results measured a quantized thermal conductance of 5/2, incompatible with the theoretical proposal. Our work revisits the theoretical approaches by pushing the expansion of the effective Hamiltonian of the 5/2 quantized Hall state to third-order in the Landau level mixing, the parameter controlling the interaction between different Landau levels. I will present the challenges behind this expansion, and our main results: the third-order expansion shows an inversion of the gaps at mixings well below the experimental regime, indicating that either the gaps are much smaller than previously expected, or that a quantum phase transition occurs. Our work also emphasizes the role of frozen spin degrees of freedom. Finally, I will discuss what is needed to give a definite answer to this long standing problem, if it is at all possible. Liens : |
| Zaki Legthas (LPENS (Paris)) | Détails Fermer |
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Magnifying Quantum Phase Fluctuations with Cooper-Pair Pairing le mardi 04 avril 2023 à 14:00 |
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Résumé : Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. Almost all these circuits operate in the regime where quantum phase fluctuations are small—the associated flux is smaller than the superconducting flux quantum—although entering the regime of large fluctuations would have profound implications for metrology and qubit protection. The difficulty arises from the apparent need for circuit impedances vastly exceeding the resistance quantum. Independently, exotic circuit elements that require Cooper pairs to form pairs in order to tunnel have been developed to encode and topologically protect quantum information. In this work, we demonstrate that pairing Cooper pairs magnifies the phase fluctuations of the circuit ground state. In a first experiment, we measure a tenfold suppression of flux sensitivity of the first transition energy only, implying a twofold increase in the vacuum phase fluctuations and showing that the ground state is delocalized over several Josephson wells. In a second experiment, we demonstrate that Cooper-pair pairing mediates high order photon-photon interactions, resulting in some peculiar spectral properties. Ref.: W. C. Smith et al., Phys. Rev. X 12, 021002 Liens : |
| Andrea Tononi Annulé | Détails Fermer |
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Self-bound fermionic mixtures in low dimensions le vendredi 31 mars 2023 à 11:00 |
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Annulé
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| Francesco Binanti (LPMMC) | Détails Fermer |
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Edge state spectroscopy of Fractional Chern Insulators le mercredi 29 mars 2023 à 11:00 |
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Résumé : ractional Chern Insulators (FCIs, which are lattice analogs of fractional quantum Hall states) have been the subject of intensive research in the last decades, not only for the interest we have in understanding the nature of topological phases of matter, but also thanks to the possible applications in quantum computing. Cold atoms in optical lattices can host these topologically ordered phases, and hallmark signatures have already been detected experimentally in two-boson systems [arXiv:2210.10919 (2022)]. We address the following question: how can we probe the edge spectrum of FCIs experimentally? We propose to subjet the atomic FCI ground state to a Laguerre- Gauss laser, creating edge excitations through a transfer of angular momentum and energy, following a similar proposal in integer Chern insulators [Phys. Rev. Lett. 108, 255303 (2012)], and to subsequently measure the excitation fraction through local density measurements. We numerically test this protocol in a model of strongly-interacting bosons in the Hofstadter lattice. We use the variation of density profile to extract the transition frequencies and consequently rebuild the excitation spectrum. We find a chiral edge branch, indicative of topological order in systems with as few as 2 bosons. Finally, we use our tool to show the progressive opening of an edge gap in the limit of very dense systems. Liens :LPMMC |
| Quentin Ficheux (Néel) | Détails Fermer |
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High-fidelity operation of fluxonium qubits le mardi 28 mars 2023 à 14:00 |
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Résumé : A promising path to reduce gate errors in superconducting quantum processors consists in developing highly anharmonic circuits with some degree of protection from prevailing decoherence sources. At present, properly designed single highly anharmonic fluxonium qubits can have over 1 ms coherence time -- about tenfold or more compared to regular transmons superconducting qubits. In this talk, I will compare the different approach to superconducting quantum computation and I will describe recent implementations of high-fidelity single and two-qubit gates in fluxonium circuits. This includes a fast microwave-activated controlled-Z gate completed in less than 9 qubit Larmor cycles (about 60 ns) with a fidelity of 99.2%, which is on-par with the best microwave-activated gates reported on several other platforms. Finally, I will discuss the prospects of extending the system size to large scale quantum processors and simulators. Liens : |
| Vittorio Vitale (LPMMC Grenoble) | Détails Fermer |
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Unsupervised learning via the Intrinsic Dimension le vendredi 24 mars 2023 à 11:00 |
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Résumé : The identification of universal properties from minimally processed datasets is one goal of machine learning techniques. Both in supervised or unsupervised settings, “making sense†of hitherto unseen raw data is defined at the outset, byencoding the task (regression, classification, etc.) in an objective function. This turns learning and inference into an optimisation problem. Here, starting from data-sets sampled from classical partition functions and one-dimensional quantum models, we build networks (graphs) by drawing links between the points according to a cutoff distance that is determined by the data structure and the choice of metric. Remarkably, this enables a transfer of methods and concepts from disconnected fields that allow us to tackle in an agnostic way the study of phase transition in several models. We observe how the minimum number of variables needed to accurately describe the important features of a data-set, the intrinsic dimension Id, behaves in the vicinity of phase transitions. We show how the finite-size analysis of the Id allows us to identify critical points with an accuracy comparable to methods that rely on apriori identification of order parameters. We review previous works [Physical ReviewX 11 (1), 011040] and elaborate on the topic with new results in case of ground states of one-dimensional quantum systems. Liens : |
| Aniket Rath (LPMMC) | Détails Fermer |
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Estimation of the quantum Fisher information using randomized measurements le mercredi 22 mars 2023 à 11:00 |
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Résumé : The quantum Fisher information (QFI) is a fundamental quantity of interest in many areas from quantum metrology to quantum information theory. It can in particular be used as a witness to establish the degree of multi-particle entanglement in quantum many-body-systems. As the QFI is a macroscopic property of the quantum state, it has been a challenge till date to measure it in state-of-art quantum platforms. To address this problem, I will firstly discuss the randomized measurement (RM) toolbox which has emerged as a good candidate to estimate properties associated to the density matrix [Nature Reviews Physics volume 5, (2023)]. This framework motivated us to formulate the QFI in terms of a converging series of lower bounds that can then be estimated using the RM toolbox [Phys. Rev. Lett. 127, 260501, (2021)]. Lastly I will show some preliminary experimental estimations of the QFI on a superconducting device that implement recent error mitigation and post-processing methods. Liens :LPMMC |
| Dominik Zumbuhl (University of Basel) | Détails Fermer |
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Building small, fast and hot hole spin qubits le mardi 21 mars 2023 à 14:00 |
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Résumé : Quantum computing holds the potential to solve key tasks exponentially faster than classical algorithms. Semiconductor spins are among the leading candidates for a highly scalable qubit platform. Hole spins offer many advantages such as all-electrical spin manipulation without micromagnets or antennas, as well as decoupling from the nuclear spins, and offer novel physics such as direct Rashba spin-orbit coupling, exchange anisotropy and longitudinal coupling which could be developed into valuable assets for quantum computing. In this seminar, I will present recent progress on two different hole spin qubit platforms: Ge/Si core/shell nanowires and Si fin field-effect transistors (FinFETs). The Ge/Si nanowires offer exceptionally strong yet electrically tunable spin-orbit interaction, the direct-Rashba SOI, allowing unprecedented control of key qubit parameters. We have recently identified sweet spots with strongly enhanced coherence, can operate the qubits at temperatures up to 2 K, are implementing an exchange based CROT 2Q gate and present dispersive qubit readout using STO varactors. Si FinFETs have championed classical transistor scaling for a decade, integrating billions of FinFETs on a chip. We have demonstrated 1Q gate fidelities at the fault-tolerance threshold at 1 K and can operate up to 5 K, potentially allowing in-situ integration of the control electronics. We have implemented a CROT gate with spin-orbit induced anisotropic exchange interaction, opening the door to high fidelity and fast 2Q gates. Recently, we have also observed phase driving of such qubits at radio frequencies. Liens :Dominik Zumbuhl |
| Igor Poboiko (Karlsruhe Institute of Technology) | Détails Fermer |
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Monitored Free Fermions and Measurements Transition le vendredi 17 mars 2023 à 11:00 |
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Résumé : We study the one-dimensionless free Fermionic model subject to random strong projective measurements of site occupation numbers, and derive an effective R-replica Keldysh non-linear sigma-model (NLSM) to study fluctuations of the entanglement entropy and fluctuations of number of particles in such a model, with the unusual replica limit R -> 1. Treatment of the NLSM within Gaussian approximation suggests logarithmic behavior of the fluctuations in the limit of rare measurements or weak monitoring, with the fluctuations growing as measurement frequency increases. The results for the transition and frequent measurements are yet to be obtained. Liens : |
| Anton Khvalyuk (LPMMC) | Détails Fermer |
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Analytical description of the superfluid stiffness in strongly disordered superconductors le mercredi 15 mars 2023 à 11:00 |
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Liens :LPMMC |
| Blagoje Oblak | Détails Fermer |
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Edge Deformations of Quantum Hall Droplets le mercredi 08 mars 2023 à 11:00 |
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Résumé : The study of two-dimensional droplets of electrons in a strong magnetic field lies at the heart of the quantum Hall effect. In this talk, I present recent results on geometric deformations of such droplets, resulting from variations of the underlying spatial metric and/or confining potential. Time-dependent variations give rise to Berry phases that can remarkably be written in closed form despite the fact that the underlying parameter space is infinite-dimensional. In particular, I argue that a large class of deformations that generalize squeezing and shearing probe the edge modes of the system, including their topological central charge. (Based on 2212.12935 and 2301.01726 , ongoing work)The study of two-dimensional droplets of electrons in a strong magnetic field lies at the heart of the quantum Hall effect. In this talk, I present recent results on geometric deformations of such droplets, resulting from variations of the underlying spatial metric and/or confining potential. Time-dependent variations give rise to Berry phases that can remarkably be written in closed form despite the fact that the underlying parameter space is infinite-dimensional. In particular, I argue that a large class of deformations that generalize squeezing and shearing probe the edge modes of the system, including their topological central charge. (Based on arXiv:2212.12935 and arXiv:2301.01726 et ongoing work) Liens : |
| Stefano Mossa (Institut de Recherche Interdisciplinaire de Grenoble (IRIG) - CEA Grenoble) | Détails Fermer |
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Statistical mechanics and simulation of nanostructure/function interplay in novel energy materials le vendredi 03 mars 2023 à 11:00 |
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Résumé : Research and development in novel energy technologies must deal with the optimization of extremely complex, often disordered, materials. On one hand, optimization calls for the development of new chemistries and materials engineering concepts; on the other hand, complexity obliges us to deeply understand fundamental physical processes, some related to a hierarchical multiscale system organization, down to nanometric sizes. At these scales, the granularity of matter cannot be put aside and the presence of interfaces, confining environments, fluctuations, crucially modify well established behaviors in the bulk. Here computer simulation and statistical mechanics approaches really help. In the talk I will overview our work in this direction, discussing a few scientific cases ranging from nanoconfined fluids [1], through polymer electrolytes [2], to superlattices and glasses [3]. Based on Molecular Dynamics simulations and artificial neural network algorithms, and also referring to data coming from advanced scattering experiments with Neutrons and X-Rays, I will describe our work to improve the understanding of disordered materials organization at the nanoscale, clarify how spatial constraint modify phase properties, and suggest how to control and optimize materials functions by tailoring the confining environments. [1] “Re-entrant phase transitions and dynamics of a nanoconfined ionic liquidâ€, [10.1103/PhysRevX.8.031062] [2] “From Ionic Surfactants to Nafion through Convolutional Neural Networksâ€, with L. Dumortier, [10.1021/acs.jpcb.0c06172] [3] “Beating the amorphous limit in thermal conductivity by superlattices designâ€, with H. Mizuno and J.-L. Barrat, [10.1038/srep14116] Liens :Stefano Mossa |
| Francesco Vercesi (LPMMC) | Détails Fermer |
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Phase diagram of 1d exciton-polariton condensate le mercredi 1er mars 2023 à 11:00 |
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Résumé : Exciton-polariton condensates represent a promising playground to investigate the physics of driven-dissipative quantum fluids. In 1d systems, the coherence of such fluids has been shown to exhibit critical behaviour, related to the Kardar-Parisi-Zhang (KPZ) universality class. Further intriguing features (phase defects, proliferation of dark solitons) have been suggested to populate the spectrum of possible regimes of exciton-polaritons. We aim to determine the phase diagram obtained via the stochastic Gross-Pitaevski equation which models the condensate at mean-field level, guided by realistic experimental tunability in the choice of the leading parameters: intensity of pumping and noise strength. Liens :LPMMC |
| Gian Marcello Andolina (Collège de France) | Détails Fermer |
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Can deep sub-wavelength cavities induce Amperean superconductivity in a 2D material? le vendredi 24 février 2023 à 11:00 |
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Résumé : Amperean superconductivity is an exotic phenomenon stemming from attractive effective electron-electron interactions (EEEIs) mediated by a transverse gauge field. Originally introduced in the context of quantum spin liquids and high-Tc superconductors, Amperean superconductivity has been recently proposed to occur at temperatures on the order of 1-20 K in two-dimensional, parabolic-band, electron gases embedded inside deep sub-wavelength optical cavities. I will first generalize the microscopic theory of cavity-induced Amperean superconductivity to the case of graphene and then argue that this superconducting state cannot be achieved in the deep sub-wavelength regime. In the latter regime, indeed, a cavity induces only EEEIs between density fluctuations rather than the current-current interactions which are responsible for Amperean pairing. Liens : |
| Tereza Vakhtel | Détails Fermer |
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Bloch oscillations in the magnetoconductance of twisted bilayer graphene le mercredi 22 février 2023 à 11:00 |
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Résumé : We identify a mapping between two-dimensional (2D) electron transport in a minimally twisted graphene bilayer and a 1D quantum walk, where one spatial dimension plays the role of time. In this mapping a magnetic field B perpendicular to the bilayer maps onto an electric field. Bloch oscillations due to the periodic motion in a 1D Bloch band can then be observed in purely DC transport as magnetoconductance oscillations with periodicity set by the Bloch frequency. Ref : Phys. Rev. B 105, L241408 (2022) Liens : |
| Tomasz Smolenski (ETH Zurich) | Détails Fermer |
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Sensing strongly correlated electrons in 2D materials le mardi 21 février 2023 à 14:00 |
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Résumé : When the strength of Coulomb interaction between itinerant electrons in a two-dimensional system becomes significantly larger than the kinetic energy, the electrons start to develop strong correlations. A paradigm phase that is expected to emerge in this regime is an electronic Wigner crystal, in which the electrons spontaneously form a periodic lattice mimicking that of atoms in the real crystals. However, in order for this electronic crystallization to occur, the actual ratio of the above energy scales must exceed 30, which turns out to be notoriously difficult to fulfill in conventional semiconductors (e.g., GaAs). Recently, atomically-thin transition metal dichalcogenides (TMDs) have emerged as a highly-tunable experimental platform that unlocks the access to uncharted territories of strongly correlated electron physics. This is due to reduced dielectric screening and large carrier effective masses, which endow TMD monolayers with excellent optical properties and give rise to strong inter-electron interactions enabling to reach Coulomb-to-kinetic energy ratios being more than an order of magnitude larger than that for the GaAs at comparable electron densities. In this talk, I will review our recent optical investigations of landmark correlated phases in charge-controlled TMD-based van der Waals heterostructures. In particular, I will present our novel spectroscopic technique enabling us to detect the Wigner crystal in a TMD monolayer through the periodic potential it generates for the excitons (1). In the presence of this potential, the excitons Bragg scatter off the Wigner crystal, which gives rise to the emergence of a Bragg-umklapp resonance in the reflectance spectrum that heralds the presence of an electronic lattice. Our observation of this resonance provides the first unequivocal evidence for the formation of the Wigner crystal that has been thus far probed only by indirect methods in two-dimensional systems. In the second part of the talk, I will also show how the Rydberg excitons in a TMD monolayer can be exploited to optically probe the formation of correlated electronic phases in an adjacent graphene layer (2), which is otherwise optically inaccessible owing to the lack of a robust energy gap. I will demonstrate that this approach allows for sensing fractional quantum Hall effect in graphene with a similar sensitivity to that of state-of-the-art transport tools. References: (1) T. Smolenski, P. E. Dolgirev, C. Kuhlenkamp, A. Popert, Y. Shimazaki, P. Back, X. Lu, M. Kroner, K. Watanabe, T. Taniguchi, I. Esterlis, E. Demler, and A. Imamoglu, Nature 595, 53-57 (2021). (2) A. Popert, Y. Shimazaki, M. Kroner, K. Watanabe, T. Taniguchi, A. Imamoglu, and T. Smolenski, Nano Letters 22, 7636 (2022) Liens : |
| MISSING (LPMMC et Néel) | Détails Fermer |
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Simulations in quantum transport and assimilation of geomagnetic data le jeudi 02 février 2023 à 14:00 |
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Résumé : Numerical simulations in the field of quantum nanoelectronics are often restricted to devices of finite size that are connected to the macroscopic world with quasi-one-dimensional electrodes. I will present a new numerical method, based on the Green’s function formalism, that lifts some of these restrictions and allows simulations of systems that are infinite in 1, 2 or 3 dimensions and mostly invariant by translation. I will illustrate these method by computing transport properties in a disorder Weyl semimetal. In a rather unrelated subject, I will talk about my work at Isterre, modelling the Earth's magnetic field and liquid iron core using geomagnetic data, i.e. satellite observations of the magnetic field. Data is cleaned from internal and external contributions, and then assimilated using an ensemble Kalman filter. Predictions of the flow are compared to changes in the length of day and show good agreement down to interannual frequencies. Liens : |
| Artem Mishchenko (Department of Physics and Astronomy, The University of Manchester) | Détails Fermer |
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Quantum transport in graphite films enabled by van der Waals technology le mardi 24 janvier 2023 à 14:00 |
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Résumé : The advent of Van der Waals technology has allowed the development of many materials that did not exist before and has led to the observation of many exciting new physical phenomena in these materials due to the unique electronic, optical, and mechanical properties of 2D atomic crystals. For instance, tuning twist angle allows altering electronic bands of 2D materials by a moiré pattern induced between 2D layers. Control of the stacking order, on the other hand, provides an alternative approach to program quantum properties, and without the need for a moiré superlattice. In this talk, I will discuss how interlayer stacking order can be used for deterministic control of the properties of van der Waals materials. In particular, controlling stacking order in multilayer graphite films allowed us to discover the quantum Hall effect in hexagonal graphite and to find strong electronic correlations in rhombohedral graphite films. Liens :Artem Mishchenko |
| Colloquium Giuseppe Carleo (EPFL - Lausanne) | Détails Fermer |
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Neural-Network Quantum States: new computational possibilities at the boundaries of the many-body problem le vendredi 20 janvier 2023 à 11:00 |
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Résumé : Machine-learning-based approaches, routinely adopted in cutting-edge industrial applications, are being increasingly adopted to study fundamental problems in science. Many-body physics is very much at the forefront of these exciting developments, given its intrinsic "big-data" nature. In this seminar I will present selected applications to the quantum realm. First, I will discuss how a systematic, and controlled machine learning of the many-body wave-function can be realized. This goal is achieved by a variational representation of quantum states based on artificial neural networks [1]. I will then discuss recent applications in diverse domains, focusing on prototypical open problems in many-body quantum physics. I will especially focus on the problem of accurately describing interacting fermions, in Condensed Matter [2], Chemistry [3], and Nuclear Matter [4] — where these approaches have significantly improved over previous variational descriptions. ————[1] Carleo and Troyer, Science 355, 602 (2017) [3] Moreno et al., PNAS 119, e2122059119 (2022) [4] Hermann et al., Nat. Chemistry 12, 891 (2020) [5] Adams et al., Phys. Rev. Lett. 127, 022502 (2021) Liens :Colloquium Giuseppe Carleo |
| Boris Brun (UGA - CEA) | Détails Fermer |
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A single hole spin with enhanced coherence in natural silicon le mardi 17 janvier 2023 à 14:00 |
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Résumé : Semiconductor spin qubits based on spin-orbit states stand as promising candidates in view of developing a quantum processor. Owing to their strong spin-orbit interaction, hole spins in silicon (1) and germanium (2) are responsive to electric field excitations, allowing for practical, fast and potentially scalable qubit control. As a drawback, spin electric susceptibility renders these qubits generally vulnerable to electrical noise, which limits their coherence time. Here we report on the operation and readout of a single hole spin in natural silicon, made from a semi-industrial 300 mm CMOS foundry. We demonstrate the existence of a preferential magnetic field orientation, at which the qubit is decoupled from charge noise while keeping an efficient electrical control. We first realize spin single-shot readout (3) of the first hole accumulated in a silicon quantum dot. Subsequently, we characterize the hole spin gyromagnetic tensor and its susceptibility to electric fields by coherent ma- nipulation techniques. We evidence a strong dependence on the external magnetic field orientation, and reveal optimal operation points at which the longitudinal spin-electric susceptibility is minimal. At these sweet spots, we measure a Hahn-Echo decay time in the order of 100 μs while maintaining Rabi frequencies in the MHz range. This work opens new perspectives for quantum processing based on spin-orbit qubits. References: (1) Piot, N., Brun, B., et al. A single hole spin with enhanced coherence in natural silicon. Nature Nanotechnology (2022). (2) Hendrickx, N. W. et al. A four-qubit germanium quantum processor. Nature 591, 580–585 (2021). (3) Elzerman, J. M. et al. Single-shot read-out of an individual electron spin in a quantum dot. Nature 430, 431–435 (2004). Liens : |
| Lorenzo Piroli (ENS Paris) | Détails Fermer |
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Quantum Cellular Automata, Tensor Networks, and Area Laws le vendredi 16 décembre 2022 à 11:00 |
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Résumé : The concept of causality, stating that physical actions cannot propagate in space at an arbitrary speed, can be captured for qudit systems by the notion of Quantum Cellular Automata (QCA), defined as unitary maps preserving locality of observables. In this talk, I will show that QCA can be identified, in any dimension and geometry, with special tensor network operators, yielding a general connection between causality and bounds on entanglement production in the form of area laws. I will stress the importance of unitarity, by discussing generalizations of our results for different classes of non-unitary quantum channels. Finally, I will mention how the set of QCA can be extended to a larger class of deterministic maps via LOCC (local operations and classical communication) and illustrate implications on state-preparation protocols and classification of phases of matter. Liens : |
| Louis Garbe (TU Wien) | Détails Fermer |
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(titre non communiqué) le mercredi 14 décembre 2022 à 11:00 |
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Liens :TU Wien |
| Jesper Nygard (Niels Bohr Institute, Univ. of Copenhagen / LANEF) | Détails Fermer |
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Superconductor-semiconductor quantum dot systems in nanowires – from in situ fabrication to entanglement le mardi 13 décembre 2022 à 14:00 |
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Résumé : Recent years superconductor-semiconductor hybrid materials have been established as an essential platform for quantum devices, notably used in the search for novel bound states that may serve as qubits (1). In this talk we firstly focus on the excitations and correlations arising in short chains of coupled quantum dots and superconducting islands (2). We then take a look beneath the surface of these nanowire devices, addressing advances in materials science where in situ MBE fabrication and various superconductors have been implemented in order to expand the available parameter space for hybrid devices (3,4,5). Ref.: (1) E. Prada et al., From Andreev to Majorana bound states in hybrid superconductor-semiconductor nanowires, Nature Reviews Physics 2, 575 (2020). (2) J.C. Estrada Saldana et al., Excitations in a superconducting Coulombic energy gap, Nature Comm. 13, 2243 (2022); Two Bogoliubov quasiparticles entangled by a spin, arxiv:2203.00104 (3) T. Kanne et al., Epitaxial Pb on InAs nanowires for quantum devices, Nature Nanotechnology 16, 767 (2021); T. Kanne et al., Double nanowires for hybrid quantum devices, Advanced Functional Materials 32, 2107926 (2021). (4) D. Carrad et al., Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids, Advanced Materials 32, 1908411 (2020). (5) J. Sestoft et al., Scalable Platform for Nanocrystal-Based Quantum Electronics, Advanced Functional Materials 32, 2112941 (2022) Liens :Jesper Nygard |
| Benjamin Wieder (IPhT, CEA, Université Paris-Saclay & MIT, Condensed Matter Theory) | Détails Fermer |
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Unraveling the Bulk and Surface Theories of Higher-Order Topological Crystalline Insulators: Axion Electrodynamics and Beyond le vendredi 09 décembre 2022 à 11:00 |
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Résumé : Topological insulating (TI) phases were originally highlighted for their disorder-robust bulk responses, such as the quantized Hall conductivity of 2D Chern insulators. With the discovery of time-reversal- (T-) invariant 2D TIs, and the recognition that their spin Hall conductivity is generically non-quantized, focus has since shifted to boundary states as signatures of 2D and 3D TIs and symmetry-enforced topological crystalline insulators (TCIs). However, in T-invariant (helical) 3D TCIs such as bismuth, BiBr, and MoTe2 - termed higher-order TCIs (HOTIs) - the boundary signatures manifest as 1D hinge states, whose configurations are dependent on sample details. It is hence desirable to elucidate bulk and surface signatures of helical TCIs, and their relationship to sample-independent experimental observables. Using a range of theoretical and numerical probes, including a newly introduced principle of spin-resolved topology, we fully characterize the bulk topological properties of inversion- and T-protected helical HOTIs. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators, "spin-Weyl" semimetal states with gapless spin spectra, and T-doubled axion insulator (T-DAXI) states with nontrivial “partial†axion angles indicative of a 3D spin-magnetoelectric bulk response. We provide experimental signatures of each spin-stable regime of helical HOTIs, including surface Fermi arcs in spin-Weyl semimetals under strong Zeeman fields, and half-quantized 2D TI states on the gapped surfaces of T-DAXIs originating from a partial parity anomaly. References: [1] K.-S. Lin, Palumbo, Z. Guo, Blackburn, Shoemaker, Mahmood, Z.-J. Wang, Fiete, Wieder, Bradlyn, arXiv:2207.10099 (2022) [2] Schindler, Tsirkin, Neupert, Bernevig, Wieder, Nature Communications (2022) Liens : |
| Rok Zitko ("Jožef Stefan" Institute) | Détails Fermer |
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Subgap states: the Richardson-model perspective le jeudi 08 décembre 2022 à 11:00 |
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Résumé : Whereas a conventional bulk superconductor is perfectly well described by the mean-field Bardeen-Cooper-Schrieffer theory of superconductivity, in the limit of ultra-small grains of superconducting material a more appropriate description is in terms of the interacting Richardson's (picket-fence) model and its extensions. This charge-conserving Hamiltonian consists of a kinetic energy term and an interacting pairing term between all orbitals, usually with a constant coefficient, in which case the model is integrable and its complete solution can be reduced to numerically solving a system of coupled algebraic equations. In the presence of an impurity carrying a local magnetic moment, the integrability is broken by the exchange scattering that splits the Cooper pairs. We show that the corresponding Hamiltonian admits a compact representation in terms of small matrix product operators, which makes it possible to solve this class of problems accurately and rather efficiently using the density matrix renormalization group (DMRG) implemented using the tensor-network formalism. I will discuss the pros and cons of the Richardson-model description of systems of coupled superconducting island and quantum dots, the effects of the Coulomb repulsion (charging) term on the superconducting island on the phase diagrams and on the nature of the (subgap) states, the basic properties of the two-channel version of the problem, as well as other possible extensions of the model (level-dependent pairing, spin-orbit coupling, two impurities). Liens :Rok Zitko |
| Antoine Browaeys (Laboratoire Charles Fabry, Institut d’Optique, CNRS) | Détails Fermer |
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Studying the many-body problem with a few assembled atoms le mardi 06 décembre 2022 à 14:00 |
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Résumé : For the last twenty years, physicists have been learning to manipulate individual quantum objects: atoms, ions, molecules, quantum circuits, electronic spins... It is now possible to build "atom by atom" a synthetic quantum matter. By controlling the interactions between atoms, one can study the properties of these elementary many-body systems: quantum magnetism, transport of excitations, superconductivity... and thus understand more deeply the N-body problem. More recently, it has been realized that these quantum machines could perhaps find applications in industrial fields, such as finding the solution of combinatorial optimization problems. Liens : |
| Andrey Zelenskiy (Dalhousie University, Canada) | Détails Fermer |
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AB-stacked Kagome Magnets: Order, Excitations, and Duality le vendredi 02 décembre 2022 à 11:00 |
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| Raphaël Menu (Universität des Saarlandes, Saarbrücken) | Détails Fermer |
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Aubry transition in a chain of trapped ions le mercredi 30 novembre 2022 à 11:00 |
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Résumé : We theoretically analyse the ground state of the Frenkel-Kontorova model when the particles interact with repulsive power-law interactions. For periodic boundary conditions we show that the classical ground state can be mapped to the one of a long-range Ising model. We then show that the classical ground state has the properties of a complete devil staircase as a function of the discommensuration and determine the parameters for which it is stable. We analyse the low-energy spectrum and discuss the effect of quantum fluctuations, focussing on experimentally relevant regimes. This work sets the stage for a full quantum mechanical description of the Aubry transition. Liens : |
| Federica Surace (Berkeley ) | Détails Fermer |
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Quantum simulation of lattice gauge theories with ultracold atoms le vendredi 25 novembre 2022 à 11:00 |
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Résumé : Gauge theories are the cornerstone of our understanding of fundamental interactions among particles. Describing the evolution of these strongly coupled systems is a formidable challenge for classical computers, and represents one of the key open quests for quantum simulation approaches to particle physics phenomena. In this talk, I will first introduce a method to realize a quantum simulation of U(1) lattice gauge theories coupled to matter, utilizing alkaline-earth(-like) atoms in state-dependent optical lattices. I will then illustrate how this quantum simulator can be used to probe various phenomena, including particle collisions and confinement. Liens : |
| Maarten Wegewijs (RWTH Aachen et FZ Jülich) | Détails Fermer |
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How quantum evolution with memory is generated in a time-local way le mercredi 23 novembre 2022 à 11:00 |
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Résumé : Various transport and optical properties of quantum devices require an account of significant coupling to their electronic or photonic environments. This makes approximation strategies necessary that go beyond the successful but limited weak-coupling, memoryless master equations (Lindblad dynamical semigroups). However, even the perturbative calculation of corrections in the environment coupling are already quite involved for stationary properties and fraught with dangers for strong local interactions. Dynamical properties further complicate the matter by requiring an account of "memory effects". In the first part of my talk I will introduce and review some of these issues from the general perspective of open-system dynamics. I will highlight how insights from quantum information about density operators can clarify, guide and motivate the application of statistical field theoretical techniques to compute their real-time evolution. In the second part of my talk I will discuss the particular situation that the same open quantum system can be equivalently described by two exact, but fundamentally different equations of motion. This puzzling issue is resolved by a surprisingly simple "fixed-point" relation between the system's memory-kernel and its generalized, time-dependent Lindblad generator. This result allows a series of long-standing problems to be solved, suggests a new general picture of memory in quantum dynamics, and provides an intriguing new iterative approach to account for memory effects. ReferenceK. Nestmann, V. Bruch, M. Wegewijs, Phys. Rev. X 11, 021041 (2021)Liens : |
| Peter Zoller (Center for Quantum Physics, University of Innsbruck - IQOQI, Austrian Academy of Sciences) | Détails Fermer |
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`Programming' Quantum Simulators with Atoms and Ions le mardi 22 novembre 2022 à 14:30 |
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Résumé : Progress in developing analog quantum simulation platforms is reflected in increasing control of engineered many-body Hamiltonians, and the ability to perform single-site and single-shot readouts. This defines a new generation of programmable quantum simulators which combine a certain amount of programmability with scalability to large particle numbers. The focus of this talk is to report work from a theory-experiment collaboration with trapped ion platforms with up to fifty qubits/spins, with the goal to develop and demonstrate quantum protocols, addressing questions from the fundamental to the practical. Examples to be discussed include measurement protocols revealing the entanglement structure of the many-body wavefunction, and implementing `optimal' quantum metrology with variational quantum circuits, where quantum simulators act as `programmable quantum sensors'. The event will be followed by a coffee in the "Salle de convivialité" close by. Liens : |
| Robert Whitney (LPMMC) | Détails Fermer |
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Illusory Cracks in the Second Law of Thermodynamics in Quantum Nanoelectronics le lundi 21 novembre 2022 à 14:00 |
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Résumé : It is easy to invent quantum systems that look like they would violate the second law of thermodynamics. Yet careful analyses — such as those reviewed here for nanoelectronic systems — have shown no such violations. Thus, to-date, cracks in the laws of thermodynamics that have sometimes been glimpsed, have always turned out to be illusory. Here I review a small subset of the domain of quantum thermodynamics by reconsidering a very old thought-experiment, usually known as Smoluchowski's trapdoor. This thought experiment was first posed by Maxwell in the 1860s, in the context of his musing on ways to violate the laws of thermodynamics; musings that led to what became known as Maxwell's demon. Smoluchowski analysed much of the physics of this trapdoor, and made a huge step towards explaining why it would not violate the laws of thermodynamics. However, I will argue that his explanation was incomplete, and that it makes violations of the laws of thermodynamics unlikely but not impossible. Now is the time to revisit this issue, because now we can use quantum dots to build an experimental nanoelectronic system that acts like a trapdoor. I analysis such a nanoelectronic trapdoor using the methods of modern quantum thermodynamics, and thereby prove that (as expected) such a trapdoor always obeys the laws of thermodynamics. The analysis of the trapdoor system is similar to that of many other nanoelectronic devices in the literature. This makes it a nice pedagogical example to guide the reader through some of the ideas and methods within quantum thermodynamics, while working on a problem that follows directly in the footsteps of the greatest thermodynamicists of the 19th century. Liens :Robert WhitneyLPMMC |
| Stefan Ilic (CSIC Donostia - San Sebastian) | Détails Fermer |
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Magnetoelectric effects and non-reciprocal transport in superconducting structures le mardi 15 novembre 2022 à 14:00 |
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Résumé : Recently, much attention is being paid to the study of superconducting systems whose transport properties depend on the direction of the supercurrents. Non-reciprocal transport effects have been proposed and observed in various structures motivated by creating a perfect superconducting diode. Besides possible applications, the physics of non-reciprocal effects is very rich, in particular in systems in which superconductivity coexists with spin-dependent fields. In this talk, I will discuss the superconducting diode effect and its connection with other closely related phenomena. These include the anomalous current and phi-Josephson junctions, the helical phase of Rashba superconductors, and magnetoelectric effects induced by the spin-orbit coupling. Liens : |
| Sylvain Ravets (C2N) | Détails Fermer |
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Driven-dissipative physics in Polariton lattices le mardi 25 octobre 2022 à 14:00 |
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Résumé : One very successful approach to photonic systems engineering is based on controlling ensembles of coupled nonlinear photonic resonators. Fascinating properties emerge at the confluence between nonlinear optics and condensed matter physics: light can undergo Bose-Einstein condensation, behave as a superfluid, or propagate along edge channels in topological lattices. One ingredient at the heart of the physics of the system is the driving field, which is used to maintain a non-equilibrium steady-state. Drive and dissipation constitute important knobs to control the physics of the system. In this talk, I will focus on two situations where the driven-dissipative nature of the system plays a key role in the physics of polariton lattices. I will start with a general introduction to polariton physics and polariton lattices [1]. I will then present two recent experiments realized at C2N in 1D lattices. In the first experiment, we investigated the non-linear optical properties of a 1D topological lattice under resonant excitation, and found pumping conditions where the nonlinear steady-state triggers the emergence of an edge state in the Bogoliubov excitation spectrum [2]. In the second experiment, we explored the physics of out-of-equilibrium Bose-Einstein condensates and evidenced universal scaling laws related to the Kardar–Parisi–Zhang universality class [3]. References: [1] C. Ciuti and I. Carusotto, Quantum fluids of light, Rev. Mod. Phys. 85, 299 (2013). [2] N. Pernet et al., Gap solitons in a one-dimensional driven-dissipative topological lattice, Nature Physics 18, 678 (2022). [3] Q. Fontaine et al., Observation of KPZ universal scaling in a one-dimensional polariton condensate, arXiv:2112.09550 (2021). Liens : |
| Willem Vos (Université de Twente) | Détails Fermer |
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Shaping waves to penetrate deep inside a forbidden gap le vendredi 21 octobre 2022 à 11:00 |
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Résumé : Liens : |
| Pavel Ostrovsky (Max Planck Institute for Solid State Research, Stuttgart) | Détails Fermer |
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Electron transport in weakly disordered Weyl semimetals le mercredi 19 octobre 2022 à 11:00 |
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Résumé : Weyl semimetal is a solid material with isolated touching points between conduction and valence bands in its Brillouin zone---Weyl points. Low energy excitations near these points exhibit a linear dispersion and act as relativistic massless particles. Weyl points are stable topological objects robust with respect to most perturbations. We study effects of weak disorder on the spectral and transport properties of Weyl semimetals in the limit of low energies. We use a model of Gaussian white-noise potential and apply dimensional regularization scheme near three dimensions to treat divergent terms in the perturbation theory. In the framework of self-consistent Born approximation, we find closed expressions for the average density of states and conductivity. Both quantities are analytic functions in the limit of zero energy. We also include interference terms beyond the self-consistent Born approximation up to the third order in disorder strength. These interference corrections are stronger than the mean-field result and non-analytic as functions of energy. Our main result is the dependence of conductivity (in units $e^2/h$) on the electron concentration $sigma = sigma_0 - 0.891 n^{1/3} et 0.115 (n^{2/3}/sigma_0) ln|n|$. Liens : |
| MISSING (Niels Bohr Institute, Univ. of Copenhagen / LANEF) | Détails Fermer |
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Superconductor-semiconductor dots and nanowires; in situ fabrication schemes and new materials for hybrid quantum devices le mardi 18 octobre 2022 à 14:00 |
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Résumé : Recent years superconductor-semiconductor hybrid materials have been established as an essential platform for quantum devices, notably used in the search for Majorana zero modes and other bound states that may serve as qubits (1). In this talk we briefly discuss hybrid quantum dot physics and then look beneath the surface of the nanowire devices, addressing the advances in materials science and nanofabrication. We will describe how in-situ fabrication (2,3,5) and various superconductors (3,4) have been implemented in order to expand the available parameter space for hybrid quantum devices. The work is mainly based on Molecular Beam Epitaxy growth of III-V nanowires, high resolution electron microscopy and low temperature electron transport experiments. References: (1) E. Prada et al., From Andreev to Majorana bound states in hybrid superconductor-semiconductor nanowires, Nature Reviews Physics (2020) (2) T. Kanne et al., Double nanowires for hybrid quantum devices, Advanced Functional Materials (2021) (3) D. Carrad et al., Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids, Advanced Materials (2020) (4) T. Kanne et al., Epitaxial Pb on InAs nanowires for quantum devices, Nature Nanotechnology (2021) (5) J. Sestoft et al, Scalable Platform for Nanocrystal-Based Quantum Electronics, Advanced Functional Materials (2022) Liens : |
| Ioan Pop (KIT) | Détails Fermer |
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High Impedance Quantum Circuits le mardi 11 octobre 2022 à 14:00 |
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Résumé : High impedance quantum circuits hold great potential for protected quantum bits and in general for coherent elements required for superconducting quantum processors. To reach the high impedance regime, we use granular Aluminum (grAl), a disordered superconductor which can be understood as a self-assembled Josephson junction array (1,2). One illustration of grAl's utility in quantum circuit design is the remarkable resilience of grAl fluxonium qubits (3,4) to photons populating its dispersively coupled readout resonator. This resilience allows single shot QND measurements (5) and quantum state preparation via active feedback with fidelity exceeding 90% even without using a parametric amplifier (6). An outstanding challenge is the mitigation of quasiparticle bursts (7) and long lived two level systems in the qubits' environment (8). References: (1) Maleeva et al. Nature Comm. 9, 3889 (2018) (2) Winkel et al. Phys. Rev. X 10, 031032 (2020) (3) Grunhaupt, Spiecker et al. Nature Materials 18, 816-819 (2019) (4) Rieger, Gunzler et al. arXiv:2202.01776 (5) Takmakov, Winkel, et al. Phys. Rev. App. 15, 064029 (2021) (6) Gusenkova, Spiecker, et al. Phys. Rev. App. 15, 064030 (2021) (7) Cardani, Valenti et al. Nat. Comm. 12, 2733 (2021) (8) Spiecker et al. arXiv:2204.00499 Liens : |
| Artem Mishenko (Manchester University) Annulé | Détails Fermer |
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(titre non communiqué) le mardi 20 septembre 2022 à 14:00 |
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Liens :Artem Mishenko |
| Audrey Bienfait (ENS Lyon) | Détails Fermer |
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Superconducting circuits for phononic quantum erasure (and for detecting spins) le mardi 13 septembre 2022 à 14:00 |
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Résumé : Heavily used in classical signal processing, surface acoustic waves (SAWs) have also been proposed as a means to coherently couple distant solid-state quantum systems. Several groups have already reported the coherent coupling of standing SAWs modes to superconducting qubits (1) In this seminar, I will describe our progress in coupling superconducting qubits to propagating SAWs. We can controllably release and capture individual itinerant photons, demonstrating that quantum state transfer as well as remote entanglement generation between superconducting qubits using phonons can be realized. Going a step further, I will show how two-phonon entanglement can also be generated and used to realize a fundamental quantum optics experiment, quantum erasure (2), using phonons (3). In a second part, I will also briefly mention my current project on how superconducting circuits can also be used for detecting spins in samples coming from condensed matter or chemical or biological applications. References: (1) M. V. Gustafsson, et al, Science, 346, 207-211, 2014 (2) M. O. Scully and K. Druhl, Phys. Rev. A 25, 2208, 1982 (3) A. Bienfait et al., Phys. Rev X 10, 021055, 2020 Liens : |
| Pavlo Sukhachov (Yale University) | Détails Fermer |
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Anomalous sound attenuation and electromagnetic field penetration in Weyl and Dirac materials le vendredi 09 septembre 2022 à 14:00 |
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Résumé : A salient feature of Weyl and Dirac materials is the possibility to realize the chiral anomaly due to their relativistic-like electronic spectra and nontrivial topology. In this seminar, I will present my recent results related to the manifestations of the chiral anomaly in sound attenuation and electromagnetic field penetration. Due to the interplay of intra- and inter-node scattering processes as well as screening, an external magnetic field generically reduces the sound absorption. A nontrivial dependence on the relative direction of the magnetic field and the sound wave vector, i.e., the magnetic sound dichroism, can occur in materials with nonsymmetric Weyl nodes. Also, I will demonstrate that the current response to an electromagnetic field in a Weyl or Dirac semimetal becomes nonlocal due to the chiral anomaly even under the conditions of the normal skin effect. Signatures of this nonlocality may be found in the transmission of electromagnetic waves. Liens : |
| Quentin Glorieux (Laboratoire Kastler Brossel) | Détails Fermer |
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Non-equilibrium physics in fluid of light: from BKT physics to turbulence le vendredi 09 septembre 2022 à 11:00 |
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Résumé : Hot atomic vapors are widely used in non-linear and quantum optics due to their large Kerr non-linearity. This non-linearity induces effective photon-photon interactions allowing light to behave as a fluid displaying quantum properties such as superfluidity. In this presentation, I will show that we have full control over the Hamiltonian that drives the system and that we can engineer an analogue simulator with light. In particular, I will discuss 2 experiments:
Liens :Laboratoire Kastler Brossel |
| Frederico Borges de (université de Sao Carlos, Brésil) | Détails Fermer |
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The internal energy of quantum systems and its additivity (room K223 and online zoom) le mardi 30 août 2022 à 14:00 |
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Résumé : In this seminar, we will address the difficulties encountered in defining the internal energy of a quantum system when studying energy exchanges in open quantum systems. This is an essential aspect of the so-called quantum thermodynamics, which claims the formulation of theoretical machinery that could be consistent for both the system of interest and its environment. Here, we will see that there is a formulation in which such consistency is naturally present and recovers the usual thermodynamic aspects of internal energy, e.g., its additivity. Liens : |
| Yu-Jie Liu (Munich Center for Quantum Science and Technology) | Détails Fermer |
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Exploiting quantum machine learning in classical and quantum tasks le mercredi 24 août 2022 à 11:00 |
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Résumé : In the first part of the talk, we discuss the usage of near-term quantum computers in solving classical tasks. The noisy-intermediate scale quantum computers are composed of a small number of qubits, and can faithfully run only short circuits. This puts many proposed approaches for quantum machine learning beyond currently available devices. We address the problem of compressing classical data into efficient representations on quantum devices. Our proposed methods allow both the required number of qubits and depth of the quantum circuit to be tuned. We achieve this by using a correspondence between matrix-product states and quantum circuits, and further propose a hardware-efficient quantum circuit approach, which we benchmark on the Fashion-MNIST dataset. Finally, we demonstrate that a quantum circuit-based classifier can achieve competitive accuracy with current tensor learning methods using only 11 qubits. In the second part of the talk, we focus on the task of classifying the quantum phases of matter using a quantum convolutional neural network (QCNN). We describe a model-independent protocol to train the QCNN. We show that similar to the definition of quantum phases, the fixed-point wavefunctions together with the unitary representation of the symmetry group of the system provide sufficient information for the QCNN to learn the structure of the phases. We test the trained QCNN on several interacting and non-interacting spin chains exhibiting trivial, symmetry-breaking, and symmetry-protected topological order. We show that the location and topology of the phase boundary are accurately predicted. Our method provides a hardware-efficient and scalable way to perform quantum phase classification on a quantum processor. Furthermore, it opens up new ways to study the quantum phases and their symmetry by exploiting classical or quantum machine learning. Liens : |
| Mucio Continentino (CBPF-Rio) | Détails Fermer |
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Thermoelectric properties of topological chains coupled to a quantum dot le vendredi 22 juillet 2022 à 11:00 |
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Résumé : Topological one-dimensional superconductors can sustain in their extremities zero energy modes that are protected by different kinds of symmetries. The observation of these excitations in the form of Majorana fermions is one of the most intensive quests in condensed matter physics. Their study is not only interesting in itself, but also because they have promising applications in the area of quantum computation. In this work we are interested in another class of one dimensional topo- logical systems, namely topological insulators. These also present symmetry protected end modes with robust properties and do not require the low temperatures necessary for topological super- conductivity. We consider the simplest kind of topological insulators, namely chains of atoms with hybridized sp orbitals. We study the transport properties of these chains in the trivial, non-trivial topological phases and at the quantum topological transition. We use a simple device consisting of two semi-infinite hybridized sp-chains connected to a quantum dot and obtain the thermoelectric properties of this system as a function of temperature and distance to the topological transition. We show that the electrical conductance and the Wiedemann-Franz ratio of the device at the topological transition have universal values at very low temperatures. The conductance and thermopower give direct evidence of fractional charges in these systems. Liens : |
| Valentin Lallemant (LPMMC) | Détails Fermer |
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(titre non communiqué) le mercredi 13 juillet 2022 à 11:00 |
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Liens :LPMMC |
| Thomas Botzung (Institute for Quantum Information, RWTH Aachen University) | Détails Fermer |
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Engineered dissipation induced entanglement transition in quantum spin chains: from logarithmic growth to area law le mercredi 06 juillet 2022 à 11:00 |
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Résumé : Recent theoretical work has shown that the competition between coherent unitary dynamics and stochastic measurements, performed by the environment, along wavefunction trajectories can give rise to novel measurement-induced phase transitions (MITs). The latter are characterized by a change in the scaling law for the entanglement entropy along quantum trajectories. First instances of such transitions have been discovered in quantum circuits of random unitaries interspersed with measurements, and subsequently also in the dynamics of other open and monitored quantum systems. Often, the competition arises between a coherent unitary evolution via a Hamiltonian or quantum gates and destructive dissipative dynamics, such as projective measurements. There, the unitary evolution drives the system towards highly entangled states while the local projective measurements partially collapse the system wavefunction and thereby reduce the entanglement. In our work, we consider a new and complementary scenario, in which it is engineered dissipative dynamics that drives the system to an entangled state, while competing Hamiltonian dynamics tends to reduce the entanglement. In this framework, we establish the existence of a MIT, and characterize its properties. As a key finding, we show that the scaling of the entanglement entropy indicates a log-to-area law transition. Liens :Thomas BotzungInstitute for Quantum Information, RWTH Aachen University |
| Gerhard Kirchmair (Innsbruck University) | Détails Fermer |
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Nonlinear Magneto-Mechanics le mardi 05 juillet 2022 à 14:00 |
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Résumé : The possibility to operate massive mechanical oscillators in the quantum regime has become central in fundamental sciences. Optomechanics, where photons are coupled to mechanical motion, provides the tools to control mechanical motion near the fundamental quantum limits. Our setup (1) consists of a magnetic field sensitive cavity coupled to a magnetic cantilever, a beam equipped with a magnet on its tip, leading to a position dependent magnetic field. A SQUID embedded in our superconducting cavity provides the sensitivity to magnetic fields. In this magneto-mechanical system, we achieve single photon coupling strength, which are among the highest in the field and more than a factor of ten larger compared to other electro-mechanical systems. Despite working at cryogenic temperatures, macroscopic mechanical objects (i.e. the cantilever) are in highly excited thermal states and need to be cooled close to the ground state in order to investigate quantum phenomena. We demonstrate a novel cooling scheme (2) by using the intrinsic nonlinearity of the cavity induced by the SQUID. We show, that the non-linearity has to be included in describing the back action and demonstrate a one order of magnitude improvement in the cooling compared to a linear system with comparable parameters. With our system it seems to be possible to overcome the back-action limit, which limits the cooling performance in linear cavities. References: (1) D. Zöpfl et al., Phys. Rev. Lett. 125, 023601 (2020); https://doi.org/10.1103/PhysRevLett.125.023601 (2) D. Zoepfl et al., arxiv:2202.13228 (2022) Liens : |
| Alexey Yamilov (Missouri University of Science & Technology) | Détails Fermer |
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Coherent control of wave propagation inside scattering media le vendredi 1er juillet 2022 à 11:00 |
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Résumé : Concept of diffusion is widely used to describe propagation of light through multiple scattering media such as clouds, interstellar gas, colloids, paint, biological tissue, etc. Such media are often called random. This terminology is, however, misleading. Notwithstanding its complexity, the process of wave propagation is entirely deterministic – uniquely defined by the exact positions of scattering centers and the shape of the incident wavefront – making it possible to deduce the precise pattern of wave field throughout the system. Technological advances over the last decade enabled one to synthesize an arbitrary wavefields opening new frontier in light control inside strongly scattering media. Feasibility of the coherent control necessitates a general framework for predicting and understanding the ultimate limit for a targeted energy delivery into a diffusive system. In this talk, we will discuss such scientifically and technologically important questions as “How can one systematically find the incident wavefront that optimally deposits energy into a target region of arbitrary size and shape, deep inside a diffusive medium?†and “What is the ultimate limit on the energy enhancement in a region?†Predictable energy delivery opens the door to numerous applications, e.g., optogenetic control of cells, photothermal therapy, as well as probing and manipulating photoelectrochemical processes deep inside nominally opaque media. Liens :Alexey Yamilov |
| Manuel Donaire (Universidad de Valladolid et Institut Néel / CNRS) | Détails Fermer |
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Excited atoms: non-reciprocal forces and optical response le mercredi 29 juin 2022 à 11:00 |
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Résumé : For atoms in their ground state, the interatomic forces and the interaction of atoms with light are well described with stationary quantum theory and semiclassical approaches. However, when atoms are excited, either by a pulse or by an incoherent pump, a fully quantum time-dependent approach becomes necessary. Within that framework, we will reveal the apparent violation of the action-reaction principle in the interaction between excited atoms, as well as the existence of non-conservative forces. In addition, we will show how to tailor the scattering properties of an atom with gains so as to obtain a PT-symmetry condition for null extinction. References
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| Mikko Möttönen (Aalto University ) | Détails Fermer |
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New superconducting qubit and millikelvin electronics to boost it le mardi 28 juin 2022 à 14:00 |
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Résumé : We recently discovered a new kind of a superconducting qubit, the unimon, that can be fabricated using standard materials and techniques out of a single Josephson junction and a superconducting resonator, yet having higher anharmonicity than the transmon and resilience against charge and flux noise. Our first experiments on the unimon demonstrate single-qubit-gate fidelity of 99.9% stable for several hours without recalibration. In addition, we have developed qubit readout, reset, and control electronics that operates at millikelvin temperatures and can be integrated with the unimon in the future. These results have been obtained by the Quantum Computing and Devices (QCD) group in collaboration with several other groups. See https://www.aalto.fi/en/department-of-applied-physics/qcd-media for highlighted published results and https://arxiv.org/abs/2203.05896 for the preprint on the unimon. ATTENTION! Séminaire en visioconférence uniquement! Liens : |
| Noam Schiller (Weizmann Institute of Science) | Détails Fermer |
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Superconductivity and fermionic dissipation in quantum Hall edges le vendredi 24 juin 2022 à 11:00 |
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Résumé : Proximity-induced superconductivity in fractional quantum Hall edges is a prerequisite to proposed realizations of parafermion zero-modes. A recent experimental work (Gül et al., arXiv: 2009.07836) provided evidence for such coupling, in the form of a crossed Andreev reflection signal, in which electrons enter a superconductor from one chiral mode and are reflected as holes to another, counter-propagating chiral mode. Remarkably, while the probability for cross Andreev reflection was small, it was stronger for $ u=1/3$ fractional quantum Hall edges than for integer ones. We theoretically explain these findings, including the relative strengths of the signals in the two cases and their qualitatively different temperature dependencies. An essential part of our model is the coupling of the edge modes to normal states in the cores of Abrikosov vortices induced by the magnetic field, which provide a fermionic bath. We find that the stronger crossed Andreev reflection in the fractional case originates from the suppression of electronic tunneling between the fermionic bath and the fractional quantum Hall edges. Liens : |
| Michael Hatridge (University of Pittsburgh) | Détails Fermer |
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Modular quantum computing and parametric controls in superconducting quantum circuits le mardi 21 juin 2022 à 14:00 |
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Résumé : Most quantum computers are built as lattices of qubits with nearest-neighbor couplings. This has several advantages: these machines are readily scaled and are well suited to error correction via surface codes. However, when operated as computers this architecture imposes a substantial overhead in implementing algorithms, as gates between distant qubits require swapping states across the lattice until they reach neighboring sites. These SWAP operations can easily dominate the gate count of the circuit, and thus limit the computational power of the quantum computer. In this talk, I will discuss our efforts to construct an alternative modular architecture for superconducting QCs via parametric gates and controls. Our scheme is based on a so-called SNAIL device whose three-wave couplings we exploit to controllably couple quantum modes. In this talk I will review our recent experimental efforts, especially our realization of four transmon all-to-all quantum modules and a quantum state router [1] which can link four modules with highly coherent operations, as well as the prospects for scaling to larger modular quantum processors. 1. A modular quantum computer based on a quantum state router C. Zhou, P. Lu, M. Praquin, T.-C. Chien, R. Kaufman, X. Cao, M. Xia, R. Mong, W. Pfaff, D. Pekker, M. Hatridge. arXiv:2109.06848 (2021). Liens :Michael Hatridge |
| MISSING (Universita di Napoli) | Détails Fermer |
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Quantum Magic le vendredi 17 juin 2022 à 14:00 |
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Résumé : Resource theories - magic and non stabilizerness - Stabilizer Renyi Entropy (SRE) - SRE and quantum chaos - measurement of magic Liens : |
| Christopher Lee Baldwin (University of Maryland) | Détails Fermer |
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Quantum dynamics in disordered systems, in low and high dimensions le vendredi 17 juin 2022 à 11:00 |
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Résumé : Quenched disorder, in the sense of random imperfections in a system, is both a blessing and a curse for scientists — it can give rise to a host of novel phenomena, but it also tends to impede transport and communication. Here we cover examples of both from our work on quantum dynamics, and in doing so touch on topics ranging from chaos to computing. In the first part, we discuss tunneling processes in “rugged energy landscapesâ€, of which the classic optimization problems from computer science (such as Traveling Salesman and Satisfiability) are examples. After describing how many such problems share essential features with mean-field spin glasses — long-range interactions, disorder, frustration — we summarize our understanding of the quantum dynamical phases in the latter. In the second part, we consider the opposite extreme of 1D nearest-neighbor spin chains. We describe how “Lieb-Robinson bounds†have proven to be an invaluable tool for studying both many-body dynamics and constraints on quantum information protocols. We then cover our recent work developing Lieb-Robinson bounds tailored to disordered spin chains. Liens : |
| Aleksey Lunkin (Landau Institute for Theoretical Physics) | Détails Fermer |
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Introduction to the SYK model and its non-Fermi liquid properties le jeudi 16 juin 2022 à 11:00 |
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Résumé : The plenty of available data on various strongly correlated electronic materials calls for the development of a general theory of the non-Fermi-liquid ground state(s) of an interacting many-body fermionic system. The recently proposed Sachdev-Ye-Kitaev (SYK) model of interacting fermions provides a new and fresh view of this old problem. It has recently attracted a lot of attention as a possible boundary theory of a two-dimensional gravitational bulk. This model, also, can be considered a nonlinear generalization of usual random-matrix Hamiltonians. In my talk, I will make an introduction to the SYK model. I will describe the mean-field solutions, its asymptotic symmetries and discuss the role of fluctuations. I also briefly show my results related to the stability of the SYK model with respect to perturbation. The last part of the talk covers transport and chaotic properties of the SYK-based model. Liens : |
| MISSING (Universita di Napoli) | Détails Fermer |
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Quantum Magic le mercredi 15 juin 2022 à 14:00 |
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Résumé : Stabilizer formalism - Clifford group - simulability of quantum states and gates Liens : |
| Martina et Félix (LPMMC) | Détails Fermer |
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Présentations des stagiaires le mercredi 15 juin 2022 à 11:00 |
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Liens : |
| Stephan Philips (TU-Delft) | Détails Fermer |
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Making quantum processors with spin qubits le mardi 14 juin 2022 à 14:00 |
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Résumé : Future quantum computers capable of solving relevant problems will require a large number of qubits that can be operated reliably(1). However, the requirements of having a large qubit count and operating with high-fidelity are typically conflicting. Spins in semiconductor quantum dots show long-term promise but demonstrations so far use between one and four qubits and typically optimize the fidelity of either single- or two-qubit operations, or initialization and readout (2,3,4,5,6,7,8). Here (9) we expand the number of qubits and simultaneously achieve respectable fidelities for universal operation, state preparation and measurement. We design, fabricate and operate a six-qubit processor with a focus on careful Hamiltonian engineering, on a high level of abstraction to program the quantum circuits and on efficient background calibration, all of which are essential to achieve high fidelities on this extended system. State preparation combines initialization by measurement and real-time feedback with quantum-non-demolition measurements. These advances will allow testing of increasingly meaningful quantum protocols and constitute a major stepping stone towards large-scale quantum computers. In this talk I will briefly review electron spin qubits and explain the results described above. 1. Vandersypen, L. M. K., et al., npj Quantum Information, vol. 3.1, pp. 1-10, 2017. 2. Veldhorst, M., et al, Nature nanotechnology, vol. 9.12, pp. 981-985, 2014. 3. Yoneda J., et al., Nature Nano, vol. 13, pp. 102-106, 2018. 4. Xue X., et al, Nature 601, 343–347, 2022 5. Noiri, A.et al., Nature 601, 338–342, 2022 6. Mills, A.et al., arXiv:2111.11937, 2021 7. Takeda K., et al., Nature Nano, pp. 1-5, 2021. 8. Hendrickx N. W., et al., Nature, vol. 591, pp. 580–585, 2021 9. Philips S., MÄ…dzik M, et al., https://arxiv.org/abs/2202.09252 Liens : |
| Alioscia Hamma (Universita di Napoli) | Détails Fermer |
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Quantum Magic le vendredi 10 juin 2022 à 14:00 |
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Résumé : Introduction - Mathematical preliminaries - layers of quantum mechanical behavior: entanglement and speed up - quantum circuits and channels - universality Liens : |
| CPTGA 10 juin (Café (Université du Maryland) | Détails Fermer |
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Scaling down the laws of thermodynamics le vendredi 10 juin 2022 à 11:00 |
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Résumé : Thermodynamics provides a robust conceptual framework and set of laws that govern the exchange of energy and matter. Although these laws were originally articulated for macroscopic objects, nanoscale systems also exhibit “thermodynamicÂ-like†behavior – for instance, biomolecular motors convert chemical fuel into mechanical work, and single molecules exhibit hysteresis when manipulated using optical tweezers. To what extent can the laws of thermodynamics be scaled down to apply to individual microscopic systems, and what new features emerge at the nanoscale? I will describe some of the challenges and recent progress – both theoretical and experimental – associated with addressing these questions. Along the way, my talk will touch on non-equilibrium fluctuations, “violations†of the second law, the thermodynamic arrow of time, nanoscale feedback control, strong system-environment coupling, and quantum thermodynamics. Liens :CPTGA 10 juin (Café |
| Richard Kueng (Johannes Kepler University Linz, Austria) | Détails Fermer |
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Classical shadows: efficient quantum-to-classical converters with many applications le jeudi 09 juin 2022 à 11:00 |
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Résumé : Extracting important information from a quantum system as efficiently and tractably as possible is an important subroutine in most quantum technologies. We present an efficient method for constructing an approximate classical description of a quantum state using very few measurements of the state. This description, called a classical shadow, can be used to predict many different properties. The required number of measurements is independent of the system size and saturates information-theoretic lower bounds [arXiv:2002.08953]. These quantum-to-classical converters pave the way for new synergies between (near-term) quantum computing and classical machine learning [arXiv:2106.12627]. Conversely, instances where they fail constitute promising candidates for new types of quantum advantage [arXiv:2112.00778]. Liens : |
| Hélène Bouchiat (LPS Orsay) | Détails Fermer |
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Singular orbital magnetism in Graphene with a moiré potential: diamagnetism and paramagnetism le mardi 07 juin 2022 à 14:00 |
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| Giacomo Mazza (Université de Genève) | Détails Fermer |
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Quantum and classical aspects of strong light-matter coupling in cavity electrodynamics le vendredi 03 juin 2022 à 11:00 |
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Résumé : Light-matter interaction represents the fundamental tool to probe and actively manipulate the properties of matter. In most cases, the investigation of strong light-matter interaction relies on coherent light sources to excite microscopic degrees of freedom. Recently, several proposals have suggested an alternative route based on the exploitation of the enhanced 'vacuum fluctuations' in confined geometries such as optical cavities. Despite the significant potential of this approach, the predictions based on simplified models of cavity QED can sometimes lead to contradictory results. In contrast to that, the properties of the light-matter interaction beyond simplified models remain poorly explored. In this seminar, I will discuss the general properties of the light-matter interaction as derived from the quantum many-body theory of photons coupled to the microscopic degrees of freedom in a solid-state system. I will consider the renormalization of electronic properties, the ground-state properties of light, and their dependence on cavity confinement. Eventually, I will compare classical and quantum aspects of the strong coupling regimes of the light-matter interaction. Liens : |
| Guillaume Manzanares (LPMMC) | Détails Fermer |
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Superradiant Quantum Phase transition for Landau Polaritons with Rashba and Zeeman couplings le mercredi 25 mai 2022 à 11:00 |
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Résumé : Cavity quantum electrodynamics has considerably developed recently thanks to the technological progress. Among the paradigmatic models of cavity quantum electrodynamics, the Dicke model, which describes the coupling of N atoms to the same cavity mode, can give rise, under some conditions, to the « superradiant » quantum phase transition (SQPT). Such a transition is predicted for a relatively high value of the light-matter coupling, which has actually been reached in the last five years in some systems, such as superconducting circuits and Landau polaritons. But while the technology is ready, the SQPT has never been observed at equilibrium because it requires also a diamagnetic energy smaller than a given threshold. I will develop a theory of cavity quantum electrodynamics for a two-dimensional electron gas in the presence of Rashba spin-orbit and Zeeman couplings and perpendicular magnetic field, coupled to a spatially nonuniform quantum photon field. I will show that the SQPT, can in principle occur through a pure in-plane Zeeman coupling, but it requires extremely small (unrealistic) quantum well widths or extremely fine tuning of the effective LandeÌ factor which makes two Landau levels coincide. Landau level crossings can also be induced by the Rashba spin-orbit coupling and they promote the SQPT which can be obtained for certain values of the effective LandeÌ factor and filling factors. In this case, the SQPT can occur for quantum well widths in the nanoscale. Liens :LPMMC |
| Saulius Vaitiekenas (Niels Bohr Institute) | Détails Fermer |
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Semi-super-ferro hybrids: A new platform for unconventional superconductivity le mardi 24 mai 2022 à 14:00 |
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Résumé : Recently developed semiconducting InAs nanowires with epitaxial superconducting Al and ferromagnetic insulator EuS shells display induced superconductivity with Zeeman-like splitting at zero external magnetic field (1). The intricate interplay between spin-orbit coupling, magnetic domains, and superconducting coherence gives rise to unique ground states and corresponding electrical properties. In this talk, I will discuss our latest experiments on spin-polarization of the induced superconductivity (2,3). References: (1) Y. Liu, et al., Nano Lett. 20, 456 (2020). (2) S. VaitiekÄ—nas, et al., Phys. Rev. B 105, L041304 (2022). (3) D. Razmadze, et al., arXiv:2204.03202 (2022). Liens : |
| Cécilia Lancien (Université Grenoble-Alpes) | Détails Fermer |
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Typical correlations and entanglement in random tensor network states le vendredi 20 mai 2022 à 11:00 |
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Résumé : Tensor network states are used extensively as a mathematically convenient description of physically relevant states of many-body quantum systems. Those built on regular lattices, i.e. matrix product states (MPS) in dimension 1 and projected entangled pair states (PEPS) in dimension 2 or higher, are of particular interest in condensed matter physics. In this talk, I will try to answer the following general question: which features of MPS and PEPS are generic and which are, on the contrary, exceptional? Or to rephrase it: given an MPS or PEPS sampled at random, what are the features that it displays with either high or low probability? One property which we will focus on is that of having either rapidly decaying or long-range correlations. In a nutshell, the main result I will state is that translation-invariant MPS and PEPS typically exhibit exponential decay of correlations, at a provably high rate. I will show two distinct ways of getting to this conclusion, depending on the dimensional regime under consideration. Both yield intermediate results which are of independent interest, namely: the parent Hamiltonian and the transfer operator of such MPS and PEPS typically have a large spectral gap. If time allows, I will also present on-going attempts at quantifying the amount of genuinely multipartite entanglement in such random MPS and PEPS. The talk will be based mainly on a joint work with David Perez-Garcia, available at arXiv:1906.11682, and on some work in progress with Ion Nechita. Liens : |
| Kater Murch (Washinton University) | Détails Fermer |
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Trapping and manipulating single-electron qubits on solid neon in a hybrid circuit quantum electrodynamics architecture le mardi 17 mai 2022 à 14:00 |
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Résumé : Electrons, elementary particles of nonzero charge, spin, and mass, have long been perceived of as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits via either motional or spin states depends critically on their material environment. I will discuss recent collaborative work where we have successfully trapped single electrons on a solid surface of neon in vacuum. By integrating an electron trap in a circuit quantum electrodynamics architecture, we achieve strong coupling between the motional states of a single electron and an on-chip microwave resonator. We further tune the system into a regime of dispersive coupling where we utilize microwave pulses to perform qubit gate operations and state readout, allowing us to characterize the coherence of this new qubit architecture. I will further discuss our plans for the next steps with his new qubit platform where we will couple to the electron’s spin which is expected to have coherence times measured in seconds. Liens : |
| MISSING (Université de Genève) | Détails Fermer |
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Influence matrix approach to quantum many-body dynamics le vendredi 13 mai 2022 à 11:00 |
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Résumé : In this talk I will introduce an approach to study the non-equilibrium dynamics of extended quantum many-body systems, inspired by the Feynman-Vernon influence functional description of quantum baths. We take an open-quantum-system viewpoint and describe evolution of a local subsystem in terms of an influence matrix (IM) - an operator acting on the space of temporal trajectories of the subsystem. The IM fully encodes the effects of the many-body system on its local subregions, and thus characterizes its ability (or failure) to behave as an efficient bath. I will show that this complementary angle of attack on quantum many-body dynamics offers many advantages, both conceptually and practically. In one spatial dimension, space-time duality allows to write an exact linear self-consistency equation for the IM. This equation possesses remarkable solutions in a class of maximally chaotic quantum circuits corresponding to perfect Markovian dephasing dynamics of subsystems. Away from such special points, quantum many-body systems exert a non-Markovian influence on subsystems, associated with temporal entanglement (TE) in the IM. Analyzing a wide range of models with analytical methods and numerical matrix-product-state computations, we study the scaling of TE in several dynamical regimes, ranging from strongly chaotic to (quasi-)integrable and many-body localized. Based on recent works with Dmitry Abanin and Michael Sonner. Liens : |
| Adrian Bachtold (ICFO) | Détails Fermer |
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Manipulating mechanical resonators with single-electron tunneling le mardi 10 mai 2022 à 14:00 |
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Résumé : Single-electron tunneling enables coupling mechanical vibrations to electrons by a large amount. In this talk, I will show how to use this coupling to create a nonlinear mechanical oscillator approaching the quantum regime, where the resulting quantum energy levels of the mechanical oscillator are no longer evenly spaced. We achieve this using carbon nanotube electromechanical resonators in the ultrastrong coupling regime, where the single-electron single-phonon electromechanical coupling can be up to 20 times larger than the mechanical frequency. Using mechanical nanotubes hosting multiple quantum dots, we expect that our approach may enable the realization of a mechanical qubit [1] and a quantum simulator of quantum matters featuring strong electron-phonon correlations [2]. [1] F. Pistolesi, A. N. Cleland, and A. Bachtold, Phys. Rev. X 11, 031027 (2021) [2] U Bhattacharya, T Grass, A Bachtold, M Lewenstein, F Pistolesi, Nano Lett. 21, 9661 (2021) Liens :Adrian Bachtold |
| Bertrand Georgeot (Laboratoire de Physique Théorique IRSAMC, Toulouse) | Détails Fermer |
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Multifractality and nonergodicity in complex quantum systems le mardi 10 mai 2022 à 11:00 |
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Résumé : Liens : |
| Marcel Filoche (LPMC, Polytechnique) | Détails Fermer |
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Is the mobility edge of the Anderson transition a percolation problem ? le vendredi 06 mai 2022 à 11:00 |
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Résumé : Anderson localization has been a intriguing phenomenon to physicists and mathematicians for now more than 6 decades. In particular, although firmly believed, the existence in 3D (and above) of a transition between localized and delocalized states has never been rigorously proven. Even more, the exact location of this transition (also called the "mobility edge") remains elusive. In this talk, we will show that a theoretical tool, the "localization landscape", casts a new light on the localization induced by a disordered potential. We will introduce the essential concepts and results obtained thanks to this tool, and show that the aforementioned mobility edge can be related to a percolation transition of the effective potential deduced from the localization landscape. We will finally present how these results can be connected to actual experimental measurements, especially in the context of cold atoms. Liens : |
| Tony Jin (Univerity of Geneva (NIGE)) | Détails Fermer |
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Classical random walker under continuous measurement le vendredi 29 avril 2022 à 11:00 |
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Résumé : The interplay between measurements and chaotic many-body systems has recently attracted extended interest within the quantum community in the context of measurement-induced phase transitions (MIPS). MIPS describes a phase transition of the entanglement entropy from a volume law to an area law when the measurement rate exceeded a certain critical value. The vast majority of the studies concerning MIPS were done for quantum systems but in principle, the interplay of measurements and chaotic dynamics could also lead to interesting phenomenology within the classical realm. In this talk, I will present one of the simplest models illustrating this interplay between an internal chaotic dynamics and a measurement process, explicitly a single random walker on the lattice undergoing continuous measurements. After presenting the model, I will show that in the limit of weak-measurement, the stochastic dynamics of the probability distribution can be mapped to the stochastic heat equation with in turn implies that the log probability follows a discrete KPZ equation via the Cole-Hopf transform. Finally, I will show numerical evidence that at higher measurement rates, a second growth regime of the width of the log probability emerges. Liens : |
| Richard East (LIG / UGA) | Détails Fermer |
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Formal diagrammatic reasoning for physics le vendredi 22 avril 2022 à 11:00 |
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| Zheng Vitto HAN (Shanxi University) | Détails Fermer |
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Substrates for graphene: a new interesting one? le mardi 12 avril 2022 à 14:00 |
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Résumé : Ever since the discovery of ultra-high mobility when placed onto an h-BN substrate, emerging physical phenomena (such as fractional quantum Hall effect, fractal Landau spectrum, and etc.) have been continuously found in graphene up to now. And along with the physical properties, the technique of transferring and stacking van der Waals layers itself has led to a new direction of research of moiré superlattice in recent years. Except for h-BN, researchers have been working on finding new substrates for graphene, in order to trigger exciting new physics in the resulting heterosystems. However, very few materials can be as potent as h-BN to serve as a substrate of graphene. In this talk, we will introduce our recent progresses in the finding of new substrates for graphene: by bring mono-layer or bernal-stacked bilayer graphene into contact with a few-layered antiferromagnetic insulator CrOCl, the resulted vertical heterostructures can give rise to an extraordinarily robust quantum Hall phase in monolayer graphene [1], and an exciton-enhanced insulator in bilayer graphene [2], which are attributed to the subtle coupling of graphene-CrOCl interface. Such interfacial coupling can be a simple yet very powerful technique in effectively engineering the quantum electronic states. [1] arXiv preprint arXiv:2110.02899. [2] arXiv preprint arXiv:2110.02921. Liens : |
| Matteo Votto (LPMMC) | Détails Fermer |
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Revealing entanglement statistics of random many-body states via partial transpose moments le vendredi 08 avril 2022 à 11:00 |
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Résumé : We present experimentally accessible quantities that can be used to identify different families of random entanglement states. In particular, we consider a ratio between low-order moments of the partially transposed reduced density matrix. We find that this ratio takes well-defined values in the thermodynamic limit for various families of entangled states. This allows to sharply distinguish each of these phases, in a way that can be understood from a quantum information perspective based on the spectrum of the partial-transpose density matrix. We analyze in particular the entanglement phase diagram of Haar random states, and the differences with respect to Clifford, matrix-product states, and fermionic Gaussian states. Our results can be used to experimentally test the mixed-state entanglement structure of quantum states formed in quantum computers and programmable quantum simulators. Liens : |
| Emmanuel Flurin (CEA Saclay) | Détails Fermer |
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Detecting spins by their fluorescence with a microwave photon counter le mardi 05 avril 2022 à 14:00 |
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Résumé : Single-photon counters are essential for detecting weak incoherent electromagnetic radiation. In the optical domain, they are widely used to detect spontaneous emission from individual quantum systems, with applications in fluorescence microscopy, and in numerous areas of quantum technologies. In the microwave domain, operational single-photon counters have just recently been developed using superconducting quantum circuits (1), offering novel opportunities for detecting fluorescence or spontaneous emission at microwave frequencies. Here, we demonstrate the use of a microwave single-photon counter to detect the photons spontaneously emitted by a small ensemble of electron spins coupled to a superconducting micro-resonator (2). In this novel spin detection scheme, each click of the detector reveals the quantum jump of an individual spin from its excited to its ground state. Besides their fundamental interest, our results also constitute a novel methodology for Electron Spin Resonance spectroscopy, it paves the way toward the readout of individual electron spins for quantum sensing at the single molecule level and quantum computation with highly coherent electron spins (3) and their nuclear registers. (1) R. Lescanne, et al., Physical Review X, 10, 021038 (2020) (2) E. Albertinale, et al., Nature 600 7889, 434-438 (2021) (3) M. Le Dantec, et al., Science advances 7.51 (2021) Liens : |
| Nicolas Bergeal (ESPCI) | Détails Fermer |
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Superconducting oxides interfaces le mardi 22 mars 2022 à 14:00 |
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Résumé : The achievement of high-quality epitaxial interfaces involving transition metal oxides gives a unique opportunity to engineer artificial materials where new electronic phases take place. The discovery of a high mobility two-dimensional electron gas (2-DEG) confined in a quantum well at the interface between two insulating oxides LaAlO3 and SrTiO3 is probably one of the most prominent examples in the field (1). Unlike more conventional semiconductor based quantum wells, conducting electrons at LaAlO3/SrTiO3 fill 3d-bands, which gives a favourable ground for the emergence of complex electronic phases. In particular, 2D superconductivity (2,3) and strong Rashba spin orbit coupling (4) have been reported in such interfaces. A key feature of these electronic systems lies in the possibility to control their carrier density by electric field effect, which results in gate-tunability of both superconductivity and Rashba spin-orbit coupling. In this talk, I will review some microwave transport measurements on LaAlO3/SrTiO3interfaces that evidence a transition from single-gap to two-gap s±-wave superconducting state driven by continuous and reversible electrostatic doping (5,6). I will also present the realization of top-gated LaAlO3/SrTiO3 devices whose physical properties, including superconductivity and Rashba spin-orbit coupling, can be tuned over a wide range of electrostatic doping, opening new perspectives for the realization of spintronics or mesoscopic devices. In particular, we have fabricated Quantum Point Contacts in an oxide interface, which exhibits a quantized conductance due to ballistic transport in a one-dimensional conducting channel. Finally, I will briefly discuss some recent experiments on the newly discovered superconducting 2-DEG in KTaO3 based heterostructures. (1) A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004). (2) A. Caviglia et al., Nature 456, 624–627 (2008). (3) J. Biscaras et al., Nature Communications 1, 89 (2010). (4) A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010). (5) G. Singh et al., Nature Mat. 18, 948–954 (2019). (6) G. Singh et al., Phys. Rev. B 105, 064512 (2022). (7) A. Jouan et al. Nature Elec. 3, 201–206 (2020). (8) C. Liu,et al. Science 371, 716–721 (2021). Z. Chen et al. Science 372, 721–724 (2021). Liens : |
| Preden Roulleau (CEA (Saclay)) | Détails Fermer |
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Excitonic nature of magnons in a quantum Hall ferromagnet le mardi 08 février 2022 à 14:00 |
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Résumé : Magnons enable the transfer of a magnetic moment or spin over macroscopic distances. In quantum Hall ferromagnets, it has been predicted that spin and charge are entangled, meaning that any change of the spin texture modifies the charge distribution. As a direct consequence of this entanglement, magnons should carry an electric dipole moment. Here we report evidence of this electric dipole moment in a graphene quantum Hall ferromagnet using a Mach-Zehnder interferometer. As magnons propagate across the insulating bulk, their electric dipole moment modifies the Aharonov-Bohm flux through the interferometer, affecting both the phase and visibility of the interference pattern. In particular, we relate the phase shift to the sign of this electric dipole moment, the loss of visibility to the flux of emitted magnons, and we show that the magnon emission is a Poissonian process. Finally, we probe the emission energy threshold of the magnons for transient states, between ν=0 and ν=1, and link them to the emergence of gapless mode predicted in the canted-antiferromagnetic phase at charge neutrality. The ability to couple the spin degree of freedom to an electrostatic potential is a property of quantum Hall ferromagnets that could be promising for spintronics. Liens : |
| Simone Rademacher (Institute for Science and Technology Austria) | Détails Fermer |
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The polaron in the strong coupling limit le vendredi 04 février 2022 à 11:00 |
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Résumé : We consider the physical system of a polaron which is a model for a charged particle moving in a polarized crystal. Its quantum mechanical description is given by the Fröhlich model, introduced in 1937. We discuss the validity of the classical Landau-Pekar equations as an effective dynamics in the strong coupling limit. Moreover, we provide a definition of the effective mass of the classical polaron described by the Landau-Pekar equations. The resulting formula agrees with the prediction by Landau and Pekar in 1948. This is joint work with D. Feliciangeli, N. Leopold, D. Mitrouskas, B. Schlein and R. Seiringer. Liens : |
| Nicolo Crescini (Néel) | Détails Fermer |
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Effect of two-level systems and phonons on superconducting quantum devices le mardi 1er février 2022 à 14:00 |
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Résumé : Two-level systems and quasiparticles are believed to lie at the origin of noise and decoherence in superconducting quantum devices. For the development of quantum technologies, it is important to understand such systems, and possibly overcome the limitation they impose. A step further in this direction is unveiling their nature, which can be discerned in terms of how they interact with micro or nano-devices. To study the physics of two-level systems (TLSs) we make use of T-shaped biased resonators. Their resonance frequency and quality factor are probed simultaneously with a microwave interferometer while applying a dc or rf bias signal. Random telegraph variations are detected in both the resonators' frequencies and quality factors, allowing us to test their correlation, and investigate the time evolution of these parameters. We observe that a milli-Volt bias can tune the resonance frequency of TLSs, while a voltage of the order of few Volts completely changes the system dynamics. Using a rf bias we perform a two-tones spectroscopy analysis, and reveal a reduction of frequency fluctuations and quality factor at low frequency and high tone power. The effect of out of equilibrium phonons is analysed by using a gated superconducting nanowire. We study the suppression of superconductivity in sample geometries where the roles of electric field and electron-current flow can be clearly separated. Our results show that suppression of superconductivity does not depend on the presence or absence of an electric field at the surface of the nanowire, but requires a current of high-energy electrons. Our observations question existing interpretations and theories based on electric fields and contribute towards understanding the complex interactions between out-of-equilibrium phenomena in solids and performance of superconducting hardware. Liens : |
| Kyrylo Snizhko (CEA Grenoble) | Détails Fermer |
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Parafermionic zero modes on quantum Hall edges, and parafermionic Kondo problem as their transport signature le mardi 25 janvier 2022 à 14:00 |
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Résumé : Fractional quantum Hall states are known to support quasiparticles that are fractions of electrons [1,2]. When combined with superconductivity, these are predicted to give rise to parafermionic zero modes — a fractional generalization of Majorana zero modes [3,4]. I will provide an introduction to the physics of parafermions, briefly describe how they can be useful for topologically protected quantum computation [5], and then discuss a parafermionic Kondo problem [6]. Parafermionic Kondo is a remarkable effect which on one hand is beautiful and counterintuitive, and on the other hand can provide strong signatures for parafermions without needing a proper quantum-computing-ready setup. [1] L. Saminadayar, D. C. Glattli, Y. Jin, and B. Etienne, “Observation of the $e/3$ Fractionally Charged Laughlin Quasiparticle,†Phys. Rev. Lett. 79, 2526–2529 (1997). [2] R. De-Picciotto, M. Reznikov, M. Heiblum, V. Umansky, G. Bunin, and D. Mahalu, “Direct observation of a fractional charge,†Nature 389, 162–164 (1997). [3] N. H. Lindner, E. Berg, G. Refael, and A. Stern, “Fractionalizing Majorana Fermions: Non-Abelian Statistics on the Edges of Abelian Quantum Hall States,†Phys. Rev. X 2, 041002 (2012). [4] D. J. Clarke, J. Alicea, and K. Shtengel, “Exotic non-abelian anyons from conventional fractional quantum Hall states,†Nat. Commun. 4, 1348 (2013). [5] K. Snizhko, R. Egger, and Y. Gefen, “Measurement and control of a Coulomb-blockaded parafermion box,†Phys. Rev. B 97, 081405 (2018). [6] K. Snizhko, F. Buccheri, R. Egger, and Y. Gefen, “Parafermionic generalization of the topological Kondo effect,†Phys. Rev. B 97, 235139 (2018). Liens : |
| Kater Murch (Washington University, St. Louis) | Détails Fermer |
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Effective non-Hermitian evolution of a superconducting qubit: harnessing the topology of a Riemann surface for quantum control le mardi 18 janvier 2022 à 14:00 |
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Résumé : A system described by a non-Hermitian Hamiltonian will, in general, have complex energies and non-orthogonal eigenstates. The degeneracies of such a system are known as exceptional points. Near these degeneracies, the complex energies are described by Riemann manifolds whose topology enables new methods of control over the system. Using a superconducting circuit QED platform we employ dynamical control over an effective non-Hermitian Hamiltonian to utilize the topology of its complex energy surfaces to control quantum state vectors. If a quantum system is initialized in one eigenstate, and the Hamiltonian parameters are varied slowly such as to encircle an EP, returning to the initial parameters, the topology of the Riemann manifold predicts that adiabatic evolution will switch the state to a different eigenstate. I will describe experiments where we observe this quantum state transport and use a quantum phase reference to measure the chiral geometric phases accumulated after this dynamical control. Liens :Kater Murch |
| Loïc Herviou | Détails Fermer |
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Time-evolution of local information: thermalization dynamics of local observables le vendredi 17 décembre 2021 à 11:00 |
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Résumé : Understanding thermalization in closed quantum mechanics remains a subtle problem. As the occupancies of the Hamiltonian eigenstates are conserved, thermalization occurs when eigenstates close in energy share similar local observables. In this talk, I will discuss thermalization through the point of view of information. I will introduce the concept of information lattice, which allows us to represent the information flow using locality as an additional dimension. Thermalization then corresponds to a flow of the state-specific information to a non-local range. Based on this observation, I will propose a tentative algorithm that allows to simulate large scale dissipative systems at long times. Liens : |
| Maria Luisa Della (Laboratoir Matériaux et Phénomènes Quantiques (MPQ)) | Détails Fermer |
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Electric and thermoelectric properties of supported 2D materials le mardi 14 décembre 2021 à 14:00 |
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Liens :Maria Luisa DellaLaboratoir Matériaux et Phénomènes Quantiques (MPQ) |
| hybride Blagoje Oblak | Détails Fermer |
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Deformational Berry Phases of Quantum Hall Droplets le vendredi 10 décembre 2021 à 11:00 |
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Résumé : In this talk, I argue that adiabatic sequences of arbitrary area-preserving deformations, acting unitarily on a planar Hall droplet, produce Berry phases that admit a closed-form expression in terms of the many- body current and density. This result is new on its own, but also exhibits several familiar concepts pertaining to the quantum Hall effect. The leading term of the many-body phase is thus super-extensive (proportional to N^2 for N electrons) owing to Aharonov-Bohm phases. Removing the latter in a gauge-invariant way leaves out an extensive geometric phase that only measures the edge current. In particular, the Berry curvature per electron associated with linear deformations is a quantized multiple of a hyperbolic area form, providing a bulk-edge correspondence for Hall viscosity. [Based on upcoming work with Benoit Estienne.] Liens : |
| Laetitia Farinacci (TU Delft) | Détails Fermer |
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Free coherent evolution of a coupled atomic spin system initialized by electron scattering le mardi 07 décembre 2021 à 14:00 |
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Résumé : Full insight into the dynamics of a coupled quantum system depends on the ability to follow the effect of a local excitation in real-time. Here, we trace the free coherent evolution of a pair of coupled atomic spins by means of scanning tunneling microscopy. Rather than using microwave pulses, we use a direct-current pump-probe scheme to detect the local magnetization after a current-induced excitation performed on one of the spins. By making use of magnetic interaction with the probe tip, we are able to tune the relative precession of the spins. We show that only if their Larmor frequencies match, the two spins can entangle, causing angular momentum to be swapped back and forth (1). These results provide insight into the locality of electron spin scattering and set the stage for controlled migration of a quantum state through an extended spin lattice. (1) Free coherent evolution of a coupled atomic spin system initialized by electron scattering, L. M. Veldman, L. Farinacci, R. Rejali, R. Broekhoven, J. Gobeil, D. Coffey, M. Ternes, A. F. Otte, Science 372, 964 (2021). Liens : |
| hybride Irénée Frérot (Institut Néel) | Détails Fermer |
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Inferring Bell's inequalities from correlation functions in quantum many-body systems le vendredi 03 décembre 2021 à 11:00 |
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Résumé : The violation of Bell's inequalities is probably the most striking manifestation of quantum entanglement, excluding all possible descriptions of a set of data in terms of a classical model (a "locally causal" model, in the words of J. S. Bell). While non-locality (that is: the violation of Bell's inequalities) has been demonstrated since the early 1980s for pairs of two-level systems, its exploration in more complex ensembles, and especially in quantum many-body systems, has remained quite unexplored until very recently. This is partly due to the lack of sufficiently scalable and flexible theoretical tools to prove the impossibility of any classical description of a given set of correlations. Yet, we will show in this talk that it is possible to devise such tools, resorting to an exact mapping between Bell's locally-causal models, and classical Ising models (or generalizations thereof). This leads to a constructive approach to probe Bell's non-locality in many-body systems, in which one tries to actively reproduce some set of observed correlations with a classical Ising model -- a so-called inverse Ising problem, well-known in many areas of data science. In contrast to typical data-science instances, however, it is the failure to find such a classical explanation to the data which ultimately leads to unveil its non-locality, in the form of new Bell's inequalities inferred from the data themselves. Both the general framework for this approach will be presented, as well as some specific results and extensions. Liens : |
| Adam Rançon (PhLAM, Lille) | Détails Fermer |
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Effective thermalization of a many-body dynamically localized Bose gas le vendredi 26 novembre 2021 à 11:00 |
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Résumé : Dynamical localization is the quantum chaos analog of the Anderson localization of disordered systems, but in momentum space, as observed in the atomic quantum kicked rotor. Interactions tend to destroy (Anderson) localization, but strong enough disorder give rise to Many-Body Localization (MBL). Whether interactions destroy dynamical localization or not is addressed here. We discuss the effect of interactions on a one-dimensional kicked Bose gas in the strong interaction limit (Tonks regime), and show that dynamical localization is not destroyed by the interactions, in the sense that the energy of the system stays finite at long times. We show that this steady-state is ergodic, i.e. described by a thermal density matrix, with an effective temperature that depends on the kicking parameters and the number of particles. The one-body reduced density matrix of the gas decays exponentially at large distance, implying absence of coherence, while the momentum distribution's tail at large momenta is characterized by an effectively thermal Tan contact. Liens : |
| Olivier Coquand (Laboratoire Charles Coulomb, Montpellier) | Détails Fermer |
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Rheology of granular liquids le mercredi 24 novembre 2021 à 11:00 |
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Résumé : Granular liquids are ubiquitous around us, from the physics of geological phenomena to the food processing industry. However, the description of their behaviour remains a challenge for fundamental physics. One of the most successful of such approaches is the so-called "µ(I)" law, a phenomenological law that describes with good accuracy experimental and numerical results, but still lacks theoretical support. In this seminar, I will present our recent works on the subject. From a set of fundamental equations, we managed to explain the µ(I) law from the competition between different time scales associated with fundamental processes within the granular flow. This shed a new light on the physics of these systems, and represents a first step towards the establishment of a complete theoretical framework to describe the physics of dense granular flows. Liens : |
| Mitali Banerjee (EPFL) | Détails Fermer |
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Identifying non-abelian anyons in quantum Hall states le mardi 23 novembre 2021 à 14:00 |
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Résumé : Quantum Hall effects in 2D electron gas exemplify the earliest class of topological phases in solid state physics, characterized by an insulating bulk and gapless edge excitations. The quantum Hall effect has been a source of many concepts that have become essential in more general quantum many-body problems. The fractional quantum Hall states host fractional charges, neutral excitations, and unlike the integer quantum Hall states, they generally support counter-propagating chiral edge modes. The theoretical and the experimental search for such fractionalized particles continues to attract extensive attention. One reason for the enthusiasm is fundamental - as fractionalization can brilliantly exemplify the rich emergent long-range behavior that many-body systems can exhibit. Another reason is more pragmatic, as certain non-abelian fractionalized excitations form the basis of topological quantum computers that promise inherent immunity against errors. In this talk, I will present the role of neutral modes in the quantum Hall effect, as the highly sought after non-abelian excitations are often “charge-neutral.†Yet, there are bosonic neutral modes that plague the single-particle interferometers in the fractional quantum Hall regime. Liens : |
| Anastasia Gorbunova | Détails Fermer |
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(titre non communiqué) le vendredi 19 novembre 2021 à 14:00 |
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| hybride Olesia Dmytruk (Collège de France) | Détails Fermer |
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Theory of high-power excitation spectra of rf-SQUID le mercredi 17 novembre 2021 à 10:00 |
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Liens : |
| Andrew Higginbotham (IST Austria) | Détails Fermer |
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Probing quantum materials with circuit quantum electrodynamics le mardi 16 novembre 2021 à 14:00 |
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Résumé : I will discuss two initial experiments in my group’s effort to probe quantum materials with circuit quantum electrodynamics. In the first experiment, we use coplanar waveguide resonators to probe superconductivity in a two-dimensional Al-InAs hybrid system. Our observations qualitatively agree with a theory of induced p±ip pairing that evolves into Bogoliubov-Fermi arcs at high magnetic field. In the second experiment, we study arrays of high-impedance Josephson junctions as a model system for the superconductor-insulator phase transition. Theory predicts that such chains should be insulating, but in experiment superconducting behavior is often observed. Fixing all parameters from microwave measurements, we find that apparent superconducting behavior can be quantitatively understood as the high-temperature fate of a melted insulator. Liens : |
| hybride Ad Lagendijk (Twente) | Détails Fermer |
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Optimizing Optical Wavefronts le lundi 15 novembre 2021 à 11:00 |
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Résumé : Liens : |
| MISSING (INRIA Grenoble et ENS-Lyon) | Détails Fermer |
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Understanding the efficiency of a nanoelectronic refrigerator fueled by a continuous quantum measurement le vendredi 12 novembre 2021 à 11:00 |
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Résumé : Recently, the booming field of quantum thermodynamics has been exploring the properties of heat engines involving phenomena specific to the quantum world, such as entanglement, coherent superpositions or quantum measurement. In particular, it has been noticed that the unavoidable perturbation induced by a quantum measurement can be seen as a thermodynamic transformation, generating a change of entropy and energy. A measuring apparatus can be seen as analogous to a heat bath, and engines able to convert the energy provided by the measurement into heat have been proposed. However, the performances of these engines cannot be completely understood without a deeper description of the energy flows involved in a quantum measurement. We perform such analysis in the case of a refrigerator fueled by a continuous measurement of charge in a nanoelectronic system. We show that for a well chosen range of parameters, the measurement-induced perturbation induces a heat flow from a cold to a hot electron reservoirs. By analyzing a realistic microscopic model for the measuring apparatus – based on an electronic tunnel junction – we compute the work and heat flows involved by the measurement and evaluate the thermodynamic efficiency of the refrigerator as a function of the parameters of the measuring apparatus. We show how the engine complies with Carnot efficiency, paving the road towards energetic optimization of quantum measurements and protocols that involve them. Liens : |
| Chuan Li (Twente University (NL)) | Détails Fermer |
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Dirac semimetal-based quantum devices towards 1D topological superconductivity le mardi 09 novembre 2021 à 14:00 |
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Résumé : One of the main directions to realize the topological quantum bit is combining topological materials with conventional superconductors. The notion of topological phases has been extended to higher-order and has been generalized to different dimensions. In the last few years, our research demonstrates the possibility of realizing the topological superconductivity in engineered 3D Dirac semimetals [1,2] and their 1D hinge states. Particularly, Cd3As2 is predicted to be a higher-order topological semimetal, possessing three-dimensional bulk Dirac fermions, two-dimensional Fermi arcs [3], and one-dimensional hinge states [4]. These topological states have different characteristic length scales in electronic transport, allowing one to distinguish their properties when changing sample size. [1] Li, C. et al. Nat. Mater. 17, 875–880 (2018). [2] Wang, A. Q. et al. Phys. Rev. Lett. 121, 237701 (2018). [3] Li, C.-Z. et al. Nat. Commun. 11, 1150 (2020). [4] Li, C.-Z. et al. Phys. Rev. Lett. 124, 156601 (2020). Liens :Twente University (NL) |
| Johannes Motruk | Détails Fermer |
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Four-spin terms and the origin of the chiral spin liquid in Mott insulators on the triangular lattice le vendredi 05 novembre 2021 à 11:00 |
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Résumé : At strong repulsion, the triangular-lattice Hubbard model is described by S=1/2 spins with nearest-neighbor antiferromagnetic Heisenberg interactions and exhibits conventional 120° order. When decreasing the interaction, additional four-spin interactions are naturally generated from the underlying Mott-insulator physics of electrons. Although these interactions have historically been connected with a gapless ground state with emergent spinon Fermi surface (SFS), we find that at physically relevant parameters, they stabilize a chiral spin-liquid (CSL) of Kalmeyer-Laughlin (KL) type, clarifying observations in recent studies of the Hubbard model. We then present a self-consistent solution based on a mean-field rewriting of the interaction to obtain a Hamiltonian with similarities to the parent Hamiltonian of the KL state, providing a physical understanding for the origin of the CSL. Using a combination of the infinite density matrix renormalization group and exact diagonalization, we also study the wider phase diagram of the spin model, shedding light on the fate of the SFS state. Finally, we will comment on experimental systems in which triangular lattice Hubbard model physics might be observed. Ref.: Phys. Rev. Lett. 127, 087201 (2021). Liens : |
| Yuli Nazarov (TU Delft) | Détails Fermer |
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Semiclassical topology in multiterminal superconducting nanostructures: protection-unprotection transition, and the role of interaction le mardi 26 octobre 2021 à 14:00 |
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Résumé : We review the conceprt of semiclassical topology in multiterminal superconducting nanostructures where the gapped states are characterzed by topological numbers. The topologicical protection requires these states to be separated by the gapless one: sometimes this work, sometimes not so a protection-unprotection transition can happen. We build up a general theoretical description of the transition vicinity in the spirit of Landau theory. We also discuss the interaction effects near the special points of the spectrum and prove a drastic effect of weak interaction. Liens : |
| Pedram Roushan (Google) | Détails Fermer |
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Time Crystals, the quest for a new phase of matter le mardi 19 octobre 2021 à 16:00 |
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Résumé : Quantum many-body systems display rich phase structure in their low-temperature equilibrium states. However, much of nature is not in thermal equilibrium. Remarkably, it was recently predicted that out-of-equilibrium systems can exhibit novel dynamical phases that may otherwise be forbidden by equilibrium thermodynamics, a paradigmatic example being the discrete time crystal (DTC). Concretely, dynamical phases can be defined in periodically driven many-body localized systems via the concept of eigenstate order. In eigenstate-ordered phases, the entire many-body spectrum exhibits quantum correlations and long-range order, with characteristic signatures in late-time dynamics from all initial states. It is, however, challenging to experimentally distinguish such stable phases from transient phenomena, wherein few select states can mask typical behavior. Here [1] we implement a continuous family of tunable CPHASE gates on an array of superconducting qubits to experimentally observe an eigenstate-ordered DTC. We demonstrate the characteristic spatiotemporal response of a DTC for generic initial states. Our work employs a time-reversal protocol that discriminates external decoherence from intrinsic thermalization, and leverages quantum typicality to circumvent the exponential cost of densely sampling the eigen-spectrum. In addition, we locate the phase transition out of the DTC with an experimental finite-size analysis. Our results establish a scalable approach to study non-equilibrium phases of matter on current quantum processors. 1. Mi et al, arXiv:2107.13571 Liens : |
| Shahal Ilani (Weizmann Institute) | Détails Fermer |
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What Is the Ultimate Conductance of Hydrodynamic Electrons? le mardi 12 octobre 2021 à 14:00 |
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Résumé : Electrical resistance usually originates from lattice imperfections. However, even a perfect lattice has a fundamental resistance limit, given by the Landauer conductance of its discrete modes. This resistance, shown by Sharvin to appear at the contacts to electronic devices, sets the ultimate conduction limit of non-interacting electrons. Recent years have seen growing evidence of hydrodynamic electronic phenomena, prompting recent theories to ask whether liquid electrons can radically break this fundamental Landauer-Sharvin limit. Here, we use single-electron-transistor imaging of electronic flows in high-mobility graphene Corbino devices to answer this question. First, by imaging ballistic flows at low temperatures, we observe a Landauer-Sharvin resistance that does not appear at contacts but is instead distributed throughout the bulk. This underpins the phase-space origin of this resistance - as emerging from spatial gradients in the number of conduction modes. At elevated temperatures, we identify and account for the contribution of electron-phonon scattering and reveal the pure hydrodynamic flow. Strikingly, we find that hydrodynamic electron flow eliminates completely the bulk Landauer-Sharvin resistance. Finally, by adding small magnetic fields, we image swirling magneto-hydrodynamic flows, revealing the key emergent length-scale predicted by hydrodynamic theories – the Gurzhi length. These observations demonstrate that electronic fluids can dramatically overcome the limitations of ballistic electrons, with important implications for fundamental science and future technologies. Liens : |
| Olivier Cepas (Institut Néel) | Détails Fermer |
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Bilan carbone de l'Institut Néel le vendredi 08 octobre 2021 à 11:00 |
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Résumé : Après un résumé sur les émissions de gaz à effet de serre et les objectifs de réduction annoncés, je vous présenterai les différentes rubriques du bilan carbone de l'Institut Néel (réalisé par le collectif "labo en transition"): missions, achats, déplacements domicile-travail, énergies, expériences sur les grands instruments, informatique etc. J'exposerai brièvement les politiques de réduction envisagées à court terme au laboratoire. Liens :Institut Néel |
| Zoltan Scherubl (CEA Grenoble / Budapest University) | Détails Fermer |
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From Cooper pair splitting to the non-local spectroscopy of a Shiba state le mardi 05 octobre 2021 à 14:00 |
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Résumé : Recent years’ hunt for the Majorana fermions generated a genuine interest in superconducting subgap (Yu-Shiba-Rusinov or shortly Shiba) states both in the field of STM measurements and hybrid semiconductor-superconductor devices. The better understanding of the trivial Shiba states paves the way to engineering them into topological states. Previously, devices, quite similar to the hybrid Majorana devices, were investigated from the viewpoint of Cooper pair splitting. These works aimed to generate spatially separated entangled electron pairs in a controlled way. In my talk, after shortly introducing these research fields, I will present a recent work, where we studied the interplay of a Shiba state and the splitting process in a Cooper pair splitter device. Changing the tunnel coupling to one of the normal electrodes allows to tune the device from limit of Cooper pair splitting to the non-local spectroscopy of the Shiba state. First, focusing on the latter limit, I will demonstrate that we could observe the Shiba state at much larger distances than in recent STM measurements. Then, I will discuss how the presence of the Shiba state, producing a non-local signal similar to the pair splitting, affects splitting efficiency. Z.S. et al., Nature Communications 11, 1834 (2020) Z.S. et al., ArXiv: 2108.12155 Liens : |
| Théotime Girardot (LPMMC) | Détails Fermer |
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Mean-field approximation for the anyon gas le mercredi 29 septembre 2021 à 14:00 |
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Résumé : This thesis is dedicated to the study of ground states of an anyon gas in the large number of particle limit. In two-dimensional space there are possibilities for quantum statistics continuously interpolating between the bosonic and the fermionic one. Quasi-particles obeying such intermediate statistics are called anyons. They can be described as ordinary bosons and fermions with interactions through long-range magnetic potentials, generated by magnetic charges carried by each particle. We study the almost-bosonic and almost-fermionic limit. We obtain rigorous results for the convergence of their ground state energies and the associated minimizers to those of effective models. The ground state of almost-bosonic anyons converges to the infimum of a Hartree-like functionnal and its minimizers to a convex superposition of factorized pure states. The particles behave like independent, identically distributed bosons interacting via a self-consistent magnetic field. The ground state energy of almost-fermionic anyons converges to the infimum of a Thomas-Fermi energy and its minimizers to measures associated with the corresponding semi-classical problem. More precisely, the ground state of our Hamiltonian converges to that of a classical modified Vlasov energy whose minimization leads to the Thomas-Fermi functional. The Vlasov energy is endowed with a self-consistent magnetic field, a landmark of anyonic statistics. The ground state of the Vlasov energy displays anyonic behavior in its momentum distribution. Liens : |
| Louk Rademaker (Department of Theoretical Physics, Université de Genève) | Détails Fermer |
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Symmetry breaking and Chern insulators in twisted graphene structures le mardi 28 septembre 2021 à 14:00 |
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Résumé : Twisted bilayer graphene (tBG) and variants like twisted monolayer-bilayer graphene (tMBG) were proposed to be a platform for strongly correlated physics akin to the cuprate family. However, I will show that many of the observed interacting phenomena can be explained in terms of breaking of spin/valley symmetry. This can lead to a quantum anomalous Hall effect in the absence of a field, as I will show for tMBG [1]. In large magnetic fields the same spin- valley symmetry breaking leads to a series of Chern insulator states [2]. Finally, I will briefly discuss the possibility of genuine strong correlated physics in Moiré structures. [1] Rademaker, Protopopov, Abanin, Phys. Rev. Research, 2, 033150 (2020). [2] Saito, Ge, Rademaker, Watanabe, Taniguchi, Abanin, Young, Nature Physics, 108, 12233 (2021). Liens :Louk Rademaker |
| Victor Bittencourt (Max-Planck Institute for the Physics of light) | Détails Fermer |
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Magnon-photon coupling in dispersive media le mercredi 22 septembre 2021 à 11:00 |
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Résumé : The quantized magnetic excitations (magnons) of magnetic dielectrics can couple to microwave and optical photons, making such systems prospect hybrid quantum platforms. Prospect applications of the system includes quantum transduction between optics and microwaves, and generation of non-classical macroscopic magnetization states. Nevertheless, one of the limitations of the system is the weakness of the coupling between magnons and optical photons - a consequence of the weakness of the magneto-optical effects in dielectrics. In this talk, I present the concept of a novel platform for magnon-photon interaction based on a magnetic epsilon-near-zero (ENZ) medium, in which strong coupling can be achieved. In ENZ media, the diagonal components of the permittivity tensor vanish at specific frequencies, yielding an enhancement of non-linear effects and secondary responses, such as magneto-optical effects. After a brief review of magnon-photon interactions in magnetic dielectrics and the state-of-the-art optomagnonic systems, I will present a quantisation scheme that phenomenologically includes dispersion, which is the responsible for the ENZ behaviour. Within this framework, the enhancement of magneto-optical effects in ENZ media implies an enhancement of the magnon-photon coupling, which can surpass both magnon and photon decay rates, thus reaching the strong coupling regime. Finally, I discuss a possible measurement scheme for the strong magnon-photon coupling via the optics power spectrum, which exhibits multiple sidebands characteristic of the strong parametric coupling. The proposal of an ENZ-based magnon-photon platform pushes forward novel designs for magnonic systems, and can be the starting point for developing new magnon-based protocols. Liens :Victor BittencourtMax-Planck Institute for the Physics of light |
| Matias Urdampilleta (Institut Néel) | Détails Fermer |
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Spin Readout in CMOS Devices le mardi 21 septembre 2021 à 14:00 |
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Résumé : Over the last fifty years, the CMOS (Complementary-Metal-Oxide-Semiconductor) electronics industry has been continuously scaling down transistors in size, to increase performance and reduce power consumption. Nowadays, the smallest transistors in industry achieve 5nm features. As a result, those silicon structures tend to exhibit undesirable quantum effects for a classical transistor which appear to be new research opportunities for quantum information processing. In particular, it is nowadays possible to trap single electron spins in silicon quantum dots and perform high fidelity quantum gates (i). These demonstrations combined with the intrinsic properties of the silicon lattice (low spin orbit and hyperfine interaction) make CMOS device an excellent candidate for scalable quantum architectures. This presentation, will focus on the probing of electronic spins trapped in a CMOS device. In the first part I will present the basic characterization of a single electron spin and valley physics detected by standard charge sensing (ii). In the second part, I will show how we can operate a small array of quantum dot (iii) and how we can measure spin states inside. Finally, I’ll present the measurement of higher spin states by probing the quantum capacitance of the system (iv) and how we intend to scale up this measurement technique. (i) Veldhorst, M. et al. Nat. Nanotechnol. 9, 981 (2014). (ii) Spence, C. et al. In preparation (2021). (iii) Chanrion, E. et al. Phys. Rev. Appl. (2020). (iv) Lundberg, T. et al. Phys. Rev. X (2020). Liens : |
| Philippe Campagne Ibarcq (INRIA/ENS Paris) | Détails Fermer |
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Quantum error correction of a qubit encoded in grid states of an oscillator le mardi 14 septembre 2021 à 14:00 |
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Résumé : In 2001, Gottesman, Kitaev and Preskill (GKP) proposed to encode a fully correctable logical qubit in grid states of a single harmonic oscillator. Although this code was originally designed to correct against shift errors, GKP qubits are robust against virtually all realistic error channels. Since this proposal, other bosonic codes have been extensively investigated, but only recently were the exotic GKP states experimentally synthesized and stabilized. These experiments relied on stroboscopic interactions between a target oscillator and an ancillary two-level system to perform non-destructive error syndrome measurements for the GKP code. In this talk, I will review the fascinating properties of the GKP code and the conceptual and experimental tools developed for trapped ions and superconducting circuits, which enabled quantum error correction of a logical GKP qubit encoded in a microwave cavity. I will describe ongoing efforts to suppress further logical errors, and in particular to avoid the apparition of uncorrectable errors stemming from the noisy ancilla involved in the GKP error syndromes detection. Liens : |
| Tobias Goecke (Deutscher Wetterdienst (DWD)) | Détails Fermer |
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Towards scale-consistent cloud models using methods from statistical physics le mercredi 08 septembre 2021 à 11:00 |
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Résumé : Methods from statistical physics have been applied to describe characteristics of cloud fields. Analogies to two-dimensional lattice models have been found, in particular for tropical convection. Some physical parametrisations of clouds make use of those ideas. But so far these parametrisations are applied in single model columns, such that horizontal correlations are mostly neglected. The analogy of cloud fields to percolation models might lend itself to describe those horizontal correlations as well as cloud size distributions. To apply those descriptions on the various scales on which physical parametrisations within weather and climate modelling are used, the models should be defined in a scale-aware manner. Here, the functional renormalization group might be a method to derive an effective model on a given scale, in order to arrive at a most consistent treatment of physical processes across models of different resolution. Liens : |
| Maxime et Robin (LPMMC) | Détails Fermer |
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(titre non communiqué) le lundi 19 juillet 2021 à 10:30 |
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Liens :LPMMC |
| Nathan et Valentin (LPMMC) | Détails Fermer |
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(titre non communiqué) le lundi 05 juillet 2021 à 10:30 |
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Liens :LPMMC |
| Florent Lecocq (NIST) | Détails Fermer |
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Hardware for efficient measurements and massive signal delivery in superconducting quantum processors le mardi 29 juin 2021 à 14:00 |
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Résumé : There are many challenges to scaling the size of superconducting quantum computers. First and foremost, building processors with increasing numbers of qubits and longer coherence remains a daunting task. However, the success of superconducting quantum computing will also hinge on the development of many supporting technologies such as cryogenics, signal delivery, and microwave readout. Here I will discuss approaches to two of these bottlenecks, lying at the interface between quantum physics and engineering. First, I will discuss the challenges of wiring a million-qubit processor with coaxial lines, and how using photonic links can enable the use of optical fibers instead [1]. Second, I will discuss why superconducting quantum processors need nonreciprocal components, what are the limitations of the conventional microwave circulators, and how we intend to replace them with integrable nonreciprocal devices based on multimode parametric interactions [2]. [1] F. Lecocq, et al, Nature 591, 575-579 (2021) [2] F. Lecocq, et al, Phys. Rev. Lett. 126, 020502 (2021) Liens :NIST |
| Salvatore Francesco Emanuele Oliviero (University of Massachussetts) | Détails Fermer |
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Quantum Chaos is Quantum le lundi 28 juin 2021 à 14:30 |
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Résumé : It is well known that a quantum circuit on n-qubits composed of Clifford gates with the addition of k non-Clifford gates can be simulated on a classical computer by an algorithm scaling polynomially in n and an exponentially in k. Our work in this direction focused on finding the number of resources needed to simulate a quantum chaotic behavior. In this talk, I will review some notions of simulability of quantum circuits and notions of Entanglement. I will show that the doping of a Clifford circuit with O(n) single qubit non Clifford resources is both necessary and sufficient to drive the transition to universal fluctuations of the purity. Liens :University of Massachussetts |
| Jonathan Atteia (Paris Sud) | Détails Fermer |
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Skyrmions and magnons in the graphene quantum Hall ferromagnet le vendredi 25 juin 2021 à 11:00 |
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Résumé : As a consequence of the approximate spin-valley symmetry in graphene, the ground state of electrons in graphene at charge neutrality is a particular SU(4) quantum Hall ferromagnet. If only the Coulomb interaction is taken into account, this ferromagnet can appeal either to the spin degree of freedom or equivalently to the valley pseudo-spin degree of freedom. This freedom in choice is then limited by subleading energy scales that explicitly break the SU(4) symmetry, the simplest of which is given by the Zeeman effect that orients the spin in the direction of the magnetic field. In addition, there are also valley symmetry breaking terms that can arise from short-range interactions or electron-phonon couplings. Here, we build upon the phase diagram in order to identify the different skyrmions at filling $ u=0$ and spin waves at $ u=pm1$ that are compatible with these types of quantum-Hall ferromagnets. Similarly to the ferromagnets, the skyrmions at charge neutrality are described by the $ ext{Gr}(2,4)$ Grassmannian at the center, which allows us to construct the skyrmion spinors. The different skyrmion types are then obtained by minimizing their energy within a variational approach, with respect to the remaining free parameters that are not fixed by the requirement that the skyrmion at large distances from their center must be compatible with the ferromagnetic background. We show that the different skyrmion types have a clear signature in the local, sublattice-resolved, spin magnetization, which is in principle accessible in scanning-tunneling microscopy and spectroscopy. In addition to the skyrmions, we also describe the generalized spin waves at $ u=pm1$, where one encounters pure spin waves, valley-pseudospin waves as well as more exotic entanglement waves that have a mixed spin-valley character. Most saliently, the SU(4) symmetry-breaking terms do not only yield gaps in the spectra, but under certain circumstances, namely in the case of residual ground-state symmetries, render the originally quadratic (in the wave vector) spin-wave dispersion linear. Liens : |
| Daniel Vert (CEA List) | Détails Fermer |
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Benchmarking quantum annealing machines for solving combinatorial problems le mardi 22 juin 2021 à 14:00 |
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Résumé : Quantum computing is receiving renewed interest with recent announcements from several players. The most obvious reason is that some quantum algorithms can solve in polynomial time the same tasks that are currently considered non-polynomial on classical computers. In recent years, analog quantum machines have appeared, of which the computers currently marketed by the Canadian company D-Wave are the first representatives, operating on a quantum accelerated annealing principle. From an abstract point of view, such a machine can be considered as an oracle specialized in solving an NP-hard optimization problem with an algorithm functionally analogous to simulated annealing but with quantum acceleration. In addition to the formal analogies between simulated annealing and quantum annealing, there is an analogy between the current state of the art and that of simulated annealing when it was introduced. The work carried out in the framework consists in understanding the functioning of such a machine and in determining to what extent it contributes to the solution of combinatorial problems. The challenge is to know whether or not there is an acceleration of a quantum nature in these machines compared to other computers. The idea is to provide a first study on the behavior of quantum annealing when confronted with a problem known to trap classical annealing. The bipartite matching problem was specifically chosen to be difficult to solve using simulated annealing. Comparing the performances between these two annealers gives a first benchmark and allows to better characterize their potentials. Liens : |
| Francesco Vercesi (LPMMC) | Détails Fermer |
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Phase transitions in 1D exciton-polariton le lundi 21 juin 2021 à 10:30 |
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Liens :LPMMC |
| Johannes Majer (University of Science and Technology of China) | Détails Fermer |
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Hybrid Quantum Systems: Coupling Diamond Color Centers to Superconducting Cavities le mardi 15 juin 2021 à 14:00 |
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Liens : |
| Silvia Viola Kusminskiy (Max Planck Institute for the science of light) | Détails Fermer |
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Quantum magnonics: Quantum optics with magnons le mardi 08 juin 2021 à 14:00 |
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Résumé : In the last five years, a new field has emerged at the intersection between Condensed Matter and Quantum Optics, denominated “Quantum Magnonicsâ€. This field strives to control the elementary excitations of magnetic materials, denominated magnons, to the level of the single quanta, and to interface them coherently to other elementary excitations such as photons or phonons. The recent developments in this field, with proof of concept experiments such as a single-magnon detector, have opened the door for hybrid quantum systems based on magnetic materials. This can allow us to explore magnetism in new ways and regimes, has the potential of unraveling quantum phenomena at unprecedented scales, and could lead to breakthroughs for quantum technologies. A predominant role in these developments is played by cavity magnonic systems, where an electromagnetic cavity, either in the optical or microwave regime, is used to enhance and control the interaction between photons and magnons. In this talk, I will introduce the field and present some theoretical results from our group which aim to push the boundaries of the current state of the art. Liens :Silvia Viola Kusminskiy |
| Alessio Chiocchetta (Köln Universität) | Détails Fermer |
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Emergent Kardar-Parisi-Zhang phase in quadratically driven condensates le lundi 07 juin 2021 à 10:30 |
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Résumé : Abstract: In bosonic gases at thermal equilibrium, an external quadratic drive can induce a Bose-Einstein condensation described by the Ising transition, as a consequence of the explicitly broken U(1) phase rotation symmetry down to Z2. However, in physical realizations such as exciton-polaritons and nonlinear photonic lattices, thermal equilibrium is lost and the state is rather determined by a balance between losses and external drive. A fundamental question is then how nonequilibrium fluctuations affect this transition. Here, we show that in a two-dimensional driven-dissipative Bose system the Ising phase is suppressed and replaced by a nonequilibrium phase featuring Kardar- Parisi-Zhang (KPZ) physics. Its emergence is rooted in a U(1)-symmetry restoration mechanism enabled by the strong fluctuations in reduced dimensionality. Moreover, we show that the presence of the quadratic drive term enhances the visibility of the KPZ scaling, compared to two-dimensional U(1)-symmetric gases, where it has remained so far elusive. Liens :Köln Universität |
| Gwendal Feve (ENS) | Détails Fermer |
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Fractional statistics of anyons in a mesoscopic collider le mardi 1er juin 2021 à 14:00 |
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Résumé : In three-dimensional space, elementary particles are divided between fermions and bosons according to the properties of symmetry of the wave function describing the state of the system when two particles are exchanged. When exchanging two fermions, the wave function acquires a phase, φ=Ï€. On the other hand, in the case of bosons, this phase is zero, φ=0. This difference leads to deeply distinct collective behaviors between fermions, which tend to exclude themselves, and bosons which tend to bunch together. The situation is different in two-dimensional systems which can host exotic quasiparticles, called anyons, which obey intermediate quantum statistics characterized by a phase φ varying between 0 and Ï€ (1,2). For example in the fractional quantum Hall regime, obtained by applying a strong magnetic field perpendicular to a two-dimensional electron gas, elementary excitations carry a fractional charge (3,4) and have been predicted to obey fractional statistics (1,2) with an exchange phase φ=Ï€/m (where m is an odd integer). I will present how fractional statistics of anyons can be demonstrated in this system by implementing and studying anyon collisions at a beam-splitter (5). The collisions are first studied in the low magnetic field regime, where the elementary excitations are electrons which obey the usual fermionic statistics. It leads to the observation of an antibunching effect in an electron collision: electrons systematically exit in two different arms of the beam-splitter. The observed result is completely different in the fractional quantum Hall regime. Fractional statistics lead to a suppression of the antibunching effect and quasiparticles tend to bunch together in larger packets of charge in a single output of the splitter. This effect leads to the observation of negative correlations of the current fluctuations (5) in perfect agreement with recent theoretical predictions (6). (1) B. I. Halperin, Phys. Rev. Lett. 52, 1583–1586 (1984). (2) D. Arovas, J. R. Schrieffer, F. Wilczek, Phys. Rev. Lett. 53, 722–723 (1984). (3) R. de Picciotto et al., Nature 389, 162–164 (1997). (4) L. Saminadayar, D. C. Glattli, Y. Jin, B. Etienne, Phys. Rev. Lett. 79, 2526–2529 (1997) (5) H. Bartolomei, M. Kumar et al. Science 368, 173-177 (2020). (6) B. Rosenow, I. P. Levkivskyi, B. I. Halperin, Phys. Rev. Lett. 116, 156802 (2016). Liens : |
| Matteo Votto (LPMMC) | Détails Fermer |
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Many-body scars and cluster Luttinger liquids in Rydberg atoms quantum simulators le lundi 31 mai 2021 à 10:30 |
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Résumé : The use of Rydberg states in order to engineer strong correlations in atomic quantum simulators has reached huge popularity in recent times, because of its flexibility in addressing different interesting regimes of quantum matter, and its push towards new interesting problems in many-body physics. An important experiment in one of these regimes showed long-lived oscillations after a quench, leading to the discovery of weak ergodicity breaking in its effective hamiltonian, then resulting in the discovery of many-body quantum scars. In this seminar, stability properties under perturbation of these peculiar states are addressed with a perturbative version of the fidelity susceptibility. Furthermore, a larger class of models with constrained dynamics is considered, showing the presence of many-body quantum scars in a broader setting. In the second part of the seminar, the focus shifts to the possibilities for equilibrium physics in a novel class of Rydberg atoms quantum simulator, based on optical lattices. Similar systems have been shown to host unconventional Luttinger liquids and exotic criticality in one dimension, in their effective descriptions, known as clustering models. Since these new setups allow for dynamics on coupled chains, quantum phases of clustering models on ladders are addressed. Liens :LPMMC |
| Jean-François Dayen (IPCMS, Strasbourg) | Détails Fermer |
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2D-0D heterostructures devices : why ? how ? what ? le mardi 25 mai 2021 à 14:00 |
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Résumé : Because of their atomically-thin structure, high surface to volume ratio, and reduced electric screening, new properties and functionalities are expected to emerge when exploiting the interactions of two dimensional (‘2D’) materials placed in contact with other nanomaterials such as zero dimensional (‘0D’) systems including clusters, nanocrystals and molecules. These so-called Mixed-dimensional van der Waals Heterostructures are now at the forefront of basic nanoscience and applied nanotechnology, providing new sets of possibilities to tailor device functions and novel physical properties.[1] Today, I will present some of our recent achievements and on-going works illustrating the possibilities offered by 2D-0D heterostructures in various fields of nanoelectronics including single-electron electronics [2], molecular electronics[3] and optoelectronics[4]. These 0D-2D devices take advantage of the functionalities of the 0D systems (electronic, magnetic, optic…) and of the specific properties of 2D materials such as : i) van der Waals interface, ii) high diffusion of metals enabling self-ordered growth of nanoclusters, iii) dual electric behavior combining in-plane charge transport with out-of-plane electric field transparency (they are thinner than the Debye screening length), iv) exacerbated surface/interface sensitivity. The works selected for this talk will allow me to introduce some of these concepts. References : [1] D. Jariwala et al., Nat. Mater. 2017, 16, 170 ; [2] Mouafo et al, Adv. Mater. 2018, 30, 1802478; Mouafo et al., Adv.Func.Mat. 2021, 31, 2008255 ; Godel et al., Adv. Mater. 2017, 29, 1604837; [3] Noumbe et al., ACS Nano 2020, 14, 4567 ; [4] Konstantinov, J. Mat. Chem. C. 2021, 9, 2712. Liens :Jean-François DayenIPCMS, Strasbourg |
| Martina Esposito (Néel) | Détails Fermer |
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A reversed Kerr travelling wave parametric amplifier: from near quantum-noise-limited amplificationto microwave photonics le mardi 18 mai 2021 à 14:00 |
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Résumé : Travelling wave parametric amplifiers (TWPAs) recently became crucial tools in superconducting quantum technologies since they allow broadband and near quantum-noise-limited microwave detection. The main obstacle in developing TWPAs is the attainment of the phase matching condition between thepump field, which provides the energy for the amplification, and the signal field, to be amplified. I will present a new experimental approach to solving the phase matching problem by exploiting a Josephson metamaterial with in-situ tunability and sign reversal of the Kerr nonlinearity: reversed Kerr phase matching. Such novel reversed Kerr TWPA (1), composed of a chain of asymmetric Josephson junction-based inductive elements, shows amplification performance superior to the ones of previous state of the art TWPAs. In this talk, I will present the design, fabrication, and amplification characterization focusing on the novel reversed Kerr phase matching method. In addition, I will show recent experimental results on the generation of two-mode squeezed statesand discuss the exciting perspective of using such a device as a source of multimode entangled states in microwave photonics. (1) Ranadive et al.A reversed Kerr traveling wave parametric amplifier(2021)http://arxiv.org/abs/2101.05815 Liens : |
| MISSING (IRIG CEA Grenoble) | Détails Fermer |
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Analog quantum simulation and quantum operations in quantum dot arrays le mardi 11 mai 2021 à 14:00 |
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| Leonardo Mazza (Université d'Orsay) | Détails Fermer |
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One dimensional Yb gases with two-body losses: strong quantum correlations in the Zeno regime le lundi 10 mai 2021 à 11:00 |
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Résumé : We consider strong two-body losses in quantum gases trapped in one-dimensional optical lattices, starting from the bosonic case. We exploit the separation of time scales typical of a system in the many-body quantum Zeno regime to establish a connection with the theory of the time-dependent generalized Gibbs ensemble. Our main result is a simple set of rate equations that capture the simultaneous action of coherent evolution and two-body losses. This treatment gives an accurate description of the dynamics of a gas prepared in a Mott insulating state and shows that its long-time behaviour deviates significantly from mean-field analyses. The possibility of observing our predictions in an experiment with 174Yb in a metastable state is also discussed. We then move to the fermionic case, where experiments with 173Yb have already been performed: our theoretical approach pinpoints the importance of spin in determining the full dynamics of the system. Liens : |
| Tobias Meng (TU Dresden) | Détails Fermer |
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Weyl semimetal black holes le jeudi 06 mai 2021 à 14:00 |
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Résumé : Weyl semimetals derive their name from the Weyl equation, the relativistic wave equation for massless spin-1/2 particles - this equation emerges as an effective low-energy theory of certain solid state compounds. This mathematical resemblance between solid state and relativity entails a number of intriguing physical parallels. One idea that has been promoted for some time is the possibility to engineer Hamiltonians that can be mapped to space-times with black holes. This talks will shed some light on what that means, and use actual lattice models to explore how far this analogy carries. Going down this road will lead us to, optimistically speaking, a poor man's version of stimulated Hawking radiation. Liens : |
| Andreas Kuhlmann (Basel University) | Détails Fermer |
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A spin qubit in a fin field-effect transistor above 4K le mardi 04 mai 2021 à 14:00 |
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Résumé : Quantum computing's greatest challenge is scaling up. Several decades ago, classical computers faced the same problem and a single solution emerged: very-large-scale integration using silicon. Today's silicon chips consist of billions of field-effect transistors (FinFETs) in which current flow along the fin-shaped channel is controlled by wrap-around gates. The semiconductor industry currently employs fins of sub-10 nm width, small enough for quantum applications: at low temperature, an electron or hole can be trapped under the gate and serve as a spin qubit. An attractive benefit of silicon's advantageous scaling properties is that quantum hardware and its classical control circuitry can be integrated in the same package. This, however, requires qubit operation at temperatures greater than 1 K where the cooling is sufficient to overcome the heat dissipation. Here, we demonstrate that a silicon FinFET is an excellent host for spin qubits that operate even above 4K. We achieve fast electrical control of hole spins with driving frequencies up to 150 MHz and single-qubit gate fidelities at the fault-tolerance threshold. The number of spin rotations before coherence is lost at these hot temperatures already matches or exceeds values on hole spin qubits at mK temperatures. While our devices feature both industry compatibility and quality, they are fabricated in a flexible and agile way to accelerate their development. This work paves the way towards large-scale integration of all-electrical and ultrafast spin qubits. Liens : |
| Anastasia Gorbunova (LPMMC) | Détails Fermer |
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(titre non communiqué) le lundi 03 mai 2021 à 10:30 |
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Liens :LPMMC |
| Peter Rickhaus (ETH Zurich) | Détails Fermer |
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Devices with Engineered Correlated States in Moire Crystals le mardi 27 avril 2021 à 14:00 |
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Résumé : When twisted to angles near 1 degre, graphene multilayers provide a new window on electron correlation physics by hosting gate-tuneable strongly-correlated states, including insulators, superconductors, and unusual magnets. I will report the discovery of a new member of the family(1), a correlated electron-hole state, in double bilayer graphene (2,3). This state can be viewed as a density wave state and is intimately linked to equilibrium exciton condensation and superfluidity. The correlated states in moiré materials can be tuned without introducing chemical dopants, thus opening the door to a new class of devices. I will show the successful realization of an exemplary device, a gate-defined Josephson Junction in magic angle twisted bilayer graphene (4). By using multilayer gate technology we define the superconducting and insulating regions of a Josephson junction (JJ) in a single-crystal and observe tunable DC and AC Josephson. This work is an initial step towards devices where gate-defined correlated states are connected in single-crystal nanostructures with applications in superconducting electronics and quantum information technology. (1) P.Rickhaus, F. K. De Vries, E. Portolés, G. Zheng, T. Taniguchi, K. Wantanabe, A. H. Macdonald, T. Ihn, and K. Ensslin, ArXiv:2005.05373 (2020). (2) F. K. de Vries, J. Zhu, E. Portolés, G. Zheng, M. Masseroni, A. Kurzmann, T. Taniguchi, K. Watanabe, A. H. MacDonald, K. Ensslin, T. Ihn, and P. Rickhaus, PRL 125 (17) (3) P. Rickhaus, G. Zheng, J. L. Lado, Y. Lee, A. Kurzmann, M. Eich, R. Pisoni, C. Tong, R. Garreis, C. Gold, M. Masseroni, T. Taniguchi, K. Wantanabe, T. Ihn, and K. Ensslin, Nano Lett. 19, 8821 (2019). (4) F. K. de Vries, E. Portolés, G. Zheng, T. Taniguchi, K. Wantanabe, T. Ihn, K. Ensslin, and P. Rickhaus, arXiv:2011.00011, accepted in Nat. Nano Liens :Peter RickhausETH Zurich |
| Aniket Rath (LPMMC) | Détails Fermer |
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Importance sampling of randomized measurements for probing entanglement le lundi 26 avril 2021 à 11:00 |
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Résumé : We show that combining randomized measurement protocols with importance sampling allows for characterizing entanglement in significantly larger quantum systems and in a more efficient way than in previous work. A drastic reduction of statistical errors is obtained using classical techniques of machine-learning and tensor networks using partial information on the quantum state. In present experimental settings of engineered many-body quantum systems this effectively doubles the (sub-)system sizes for which entanglement can be measured. In particular, we show an exponential reduction of the required number of measurements to estimate the purity of product states and GHZ states. Liens :LPMMC |
| Williams Savero Torres (ICN2, Barcelona) | Détails Fermer |
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“Spin orbit phenomena in graphene-based heterostructures le mardi 06 avril 2021 à 14:00 |
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Résumé : In the last years, van der Waals heterostructures have attracted an increasing attention due to their outstanding properties, resulting from combining different two dimensional materials in compact structures, that have led to the emergence of new phenomena not accesible in other platforms (1). Among them, graphene constitutes a promissing material for spintronics because it enables the transport of spin signals over larger distances compared to other systems (2). However, its low intrinsic spin-orbit coupling difficults spin signal manipulation, which has prevented the fast application of graphene for spintronic devices. In this seminar, I will describe two of our recent studies performed in graphene-based heterostructures, where we demonstrate that spin signals can be generated and manipulated by means of proximity effects induced by spin-orbit phenomena. In the first part, I will show how the imprinted spin texture in graphene interfaced with a transition metal dichalcogenide give rise to an anisotropic spin relaxation, where the spin lifetime for spins oriented out-of-plane is one order of magnitude larger than those oriented in-plane (3). In the second part, I will show how such proximity-induced effects can be used to generate spin signals in graphene that can also be controlled by electrical gatting with one of the highest efficiency reported to date at room temperature (4). These results provide the building blocks for development of ultra-compact devices made of two dimensional materials. (1) W. Savero Torres et al. MRS Bulletin 45(5), 357-365, (2020) (2) W. Savero Torres et al. 2D Mat. 4, 041008, (2017) (3) L. A. Benítez, J.F. Sierra, W. Savero Torres et al. Nat. Phys. 14, 303-308, (2018) (4) L. A. Benítez, W. Savero Torres et al. Nat. Mat. 19, 170-175, (2020) Liens :ICN2, Barcelona |
| Matthieu Dartiailh (NYU) | Détails Fermer |
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Phase Signature of Topological Transition in Josephson Junctions le mardi 30 mars 2021 à 14:00 |
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Résumé : Topological transition transforms common superconductivity into an exotic phase of matter, which holds promise for fault tolerant quantum computing. A hallmark of this transition is the emergence of Majorana states. While two dimensional semiconductor superconductor heterostructures are desirable platforms for topological superconductivity, direct phase measurements as the fingerprint of the underlying topological transition are conspicuously missing. On gate tunable Josephson junctions made on epitaxial Al InAs, we observe a closing and a reopening of the superconducting gap with increasing in plane magnetic field. Since our junctions are embedded into a phase sensitive SQUID, we are able to measure a pi jump in the superconducting phase across the junction coincident with the closing and reopening of the superconducting gap. Theoretical simulations confirm this transition is topological and compatible with the emergence of Majorana states while the magnetic field angle dependence of the transition further constrain this scenario. Remarkably, in each junction, this topological transition can be controlled by changing the gate voltage. These findings reveal versatile two dimensional platforms for scalable topological quantum computing. Liens : |
| Clemens Winkelmann (Inst. Neel) | Détails Fermer |
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Heat transport and thermopower in strongly coupled single quantum dot devices le mardi 23 mars 2021 à 14:00 |
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Résumé : We experimentally and theoretically investigate the thermoelectric properties of gate-tunable single-quantum dot junctions at milliKelvin temperatures, with particular focus on the strong tunnel coupling regime, Γ > k_BT. In the spin-1/2 Kondo regime of the quantum dot, we have observed that the Seebeck coefficient displays characteristic sign changes with varying temperature and level depth, in good agreement with numerical renormalization group calculations [1]. We then move to the question of heat transport across a single quantum level. While the heat conductance of a sequentially coupled single quantum level is expected to be uniformly equal to zero, we show both experimentally and theoretically, that the inclusion of cotunnelling effects leads to restoring a finite heat conductance [2]. [1] B. Dutta et al., Nano Lett. 19, 506 (2019). [2] B. Dutta et al., Phys. Rev. Lett. 125, 237701 (2020). Liens :Clemens WinkelmannInst. Neel |
| Giovanni Pecci (LPMMC) | Détails Fermer |
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Spin fluctuations dynamics in harmonically trapped Fermi gases at strong interactions le lundi 22 mars 2021 à 11:00 |
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Résumé : We study the dynamics of a two spin component one-dimensional Fermi gas trapped in an harmonic potential. We focus on the strongly interacting regime, performing a perturbative analysis starting from the regime of infinite interactions, where the model can be solved exactly. In this regime, spin and density degrees of freedom decouple and evolve independently in time. We consider an out-of-equilibrium initial state where the spin up and spin down particles are spatially separated in the trap. In this case, the dynamics of the particle density is trivial, while the single-spin component densities oscillate in time. We address the dynamics in the spin sector, performing numerical diagonalization of the Hamiltonian for different number of particles and comparing the different frequencies of the spin oscillations. Liens :LPMMC |
| Michele Filippone (CEA Grenoble) | Détails Fermer |
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Quantum simulation with solid-state quantum technologies : Observing many-body localization in a superconducting qubit array le mardi 16 mars 2021 à 14:00 |
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Résumé : In this seminar, I will discuss how quantum technologies are now able to unveil and investigate novel fundamental phenomena by simulating interacting quantum systems. In particular, we will ask : Is it possible to harness and preserve the quantum coherent properties of many-body systems?This project seems doomed to fail, as interactions in many-body systems generally lead to ergodicity, namely the inevitable loss of quantum coherence and memory about initial conditions. Nevertheless, the recent discovery of many-body localization (MBL) – a generalization of Anderson localization in the presence of interactions – has shown the possibility to circumvent ergodicity. I will illustrate an experiment in which an array of superconducting qubits probes the exotic dynamics of interacting and disordered bosons (1). Relying on real-time and interferometric probes, I will discuss how we could observe and characterize the mechanism of MBL. (1) https://arxiv.org/abs/1910.06024 Liens : |
| Jonathan Wise (LPMMC) | Détails Fermer |
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Near field versus far field in radiative heat transfer between two-dimensional metals le lundi 15 mars 2021 à 11:00 |
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Résumé : Using the standard fluctuational electrodynamics framework, we analytically calculate the radiative heat current between two thin metallic layers, separated by a vacuum gap. We analyse different contributions to the heat current (travelling or evanescent waves, transverse electric or magnetic polarization) and reveal the crucial qualitative role played by the dc conductivity of the metals. For poorly conducting metals, the heat current may be dominated by evanescent waves even when the separation between the layers greatly exceeds the thermal photon wavelength, and the coupling is of electrostatic nature. For well-conducting metals, the evanescent contribution dominates at separations smaller than the thermal wavelength and is mainly due to magnetostatic coupling, in agreement with earlier works on bulk metals. Liens :LPMMC |
| Blagoje Oblak | Détails Fermer |
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Berry phases and drift in the KdV equation le lundi 1er février 2021 à 10:30 |
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Résumé : I consider a model of fluid particle motion closely related to the Korteweg-de Vries equation governing shallow water dynamics. Using the reformulation of this model as a geodesic in an infinite-dimensional group, the drift velocity of particles is shown to be an ergodic rotation number, sensitive to Berry phases produced by adiabatic spatial deformations. Along the way, I show that the topology of coadjoint orbits of wave profiles affects drift in a dramatic manner: orbits that are not homotopic to a point yield quantized rotation numbers. These arguments rely on the general structure of Euler equations, suggesting the existence of other similar applications of infinite-dimensional geometry to nonlinear waves. |
| Tommaso Comparin (ENS-Lyon) | Détails Fermer |
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Quench Spectroscopy: Low-energy excitations from real-time quantum dynamics le lundi 25 janvier 2021 à 10:30 |
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Résumé : Experiments with quantum simulators (made of ultracold atoms, trapped ions, etc.) are not only a way to realize low-energy states of quantum matter, but they also offer an unprecedented set of tools to control and analyze quantum dynamics. For systems that are well isolated from their environment, such dynamics is fully determined by the many-body Hamiltonian. In our theoretical study we consider spin models with power-law couplings; for small systems we describe the dynamics exactly, while we employ the time-dependent Variational Monte Carlo technique to treat larger sizes. We first focus on Quench Spectroscopy, an approach to characterize low-energy excitations on top of the ground state. Starting from an uncorrelated state, spin-spin correlations appear and propagate during time evolution. Their spectral analysis (in momentum and frequency) shows signatures of low-energy excitations, like the quasiparticles described by linear spin waves. Quench Spectroscopy is an alternative to traditional spectroscopy approaches, as those implemented through inelastic neutron scattering or Bragg spectroscopy. As a second application, we look at the dynamical signatures of a subtle property of the energy spectrum, namely the presence of Anderson's tower of states. These states are connected to the eigenstates of a simpler model, consisting of a large-spin rigid rotor. We show how this link can be unveiled in the time evolution of collective spin variables, and how it gives us information about the generation of squeezing during the dynamics. Liens : |
| Jan Behrends (Cambridge University) | Détails Fermer |
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(Super)symmetries in the Sachdev-Ye-Kitaev model le vendredi 22 janvier 2021 à 11:00 |
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Résumé : The Sachdev-Ye-Kitaev (SYK) model is an interaction-only toy model for quantum chaos, holography and non-Fermi liquids. In its simplest form, Majorana fermions interact via structureless all-to-all four-body interactions. In this talk, I will demonstrate that this system belongs to one of eight (real) Altland-Zirnbauer symmetry classes set only by the number of Majorana fermions. We show that, depending on the symmetry class, the system may support exact many-body zero modes. These are manifestations of an intrinsic supersymmetry that requires no relations between couplings, in contrast to existing explicitly supersymmetric extensions of the model. The supersymmetry we uncover has a natural interpretation in terms of a one-dimensional topological phase supporting Sachdev–Ye–Kitaev boundary physics and has consequences away from the ground state, including in q-body dynamical correlation functions. Finally, I will briefly talk about a dynamical protocol based on Majorana zero modes that realize SYK physics. |
| Tudor-Alexandru Petrescu (Université de Sherbrooke) | Détails Fermer |
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Readout problem in circuit QED: drive-induced enhancement to the Purcell effect and other nonlinear relaxation mechanisms le lundi 18 janvier 2021 à 14:30 |
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Résumé : With current advances in state preparation, as well as gate and measurement operations, superconducting circuits are now a leading architecture for quantum information processing. As these systems are scaled up, strict requirements on the fidelity of operations required for computation and readout are imposed. In this talk we focus on the so-called “readout problem†in circuit quantum electrodynamics: several experiments have shown that qubit energy relaxation rates may become strongly dependent on the power of the measurement drive, even for moderate or weak drives; this hampers efforts to improve readout fidelity. To explain this, we devised a perturbation theory for driven-dissipative, weakly anharmonic, superconducting circuits based on a sequence of unitary transformations. Applied to a transmon qubit coupled to a readout resonator, this approach allows us to classify the nonlinear processes that enhance qubit relaxation in the presence of resonator photons. Among these, we are able to quantify changes to the Purcell rate, and to stimulated emission. Chiefly responsible for the dressing of relaxation rates are the counterrotating terms arising from the expansion of the Josephson potential, which are usually neglected in theories based on Kerr nonlinear oscillators. Time- permitting, we will discuss a general framework for quantizing driven superconducting circuits that arises from this study, with a concrete example in the accurate modeling of two-qubit gates. Liens :Université de Sherbrooke |
| Adriano Angelone (ICTP Trieste) | Détails Fermer |
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Strongly Correlated Systems of Bosons and Fermions: Many-body phenomena and Numerical Methods le lundi 11 janvier 2021 à 10:30 |
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Résumé : Many interesting physical phenomena are connected to strongly correlated systems, which, due to their complexity, cannot usually be studied analitically, making numerical approaches essential. The development of the latter and the study of the physical scenarios induced by strong correlations are therefore both of great importance: in my talk, I will present my results in the context of both these research directions. I will discuss my Path Integral Monte Carlo (MC) results about the equilibrium and out-of-equilibrium physics of a class of lattice bosonic models with extended-range interactions, relevant for experiments with cold Rydberg and Rydberg-dressed atoms, where exotic phenomena such as supersolid-supersolid transitions and (super)glass phases are induced by the formation of particle clusters in the medium- and strong-interaction regime. Furthermore, I will present my work on the ground-state properties of the fermionic t-J model, a candidate Hamiltonian to describe high-T_c superconductivity, in the presence of two mobile holes. Here, I employ Variational MC in conjunction with the Entangled Plaquette States (EPS) ansatz, a versatile and powerful trial wavefunction for the study of fermionic and frustrated many-body systems. My results confirm existing predictions with much higher accuracy and (unlike previously) to sizes large enough to approximate well the thermodynamic limit, and are foundational to prove the applicability of the EPS ansatz to other computationally challenging many-body problems. Liens : |
| Pierre Nataf (LPMMC) | Détails Fermer |
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Numerical methods to investigate Heisenberg SU(N) lattice models le lundi 14 décembre 2020 à 10:30 |
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Résumé : Systems of multicolor fermions have recently raised considerable interest due to the pos-
sibility to experimentally study those systems on optical lattices with ultracold atoms [1].
To describe the Mott insulating phase of N-color fermions, one can start with the SU(N)
Heisenberg Hamiltonian. In the case of one particule per site, the SU(N) Heisenberg Hamiltonian takes the form of a Quantum permutation Hamiltonian H = J ∑〈ij〉Pij , where the
transposition operator Pij exchanges two colors on neighboring sites.
We have developped a method[2] to implement the SU(N) symmetry in an Exact Diagonalization algorithm. In particular, the method enables one to diagonalize the Hamiltonian
directly in the irreducible representations of SU(N), thanks to the use of standard Young
tableaux[3], which are shown to form a very convenient basis to diagonalize the problem. It
allowed us to prove that the ground state of the Heisenberg SU(5) model on the square lattice is long range color ordered [2] and it provided evidence that the phase of the Heisenberg SU (6) model on the Honeycomb lattice is a plaquette phase [4]. Finally, we have general-
ized the method to Density Matrix Renormalization Group [5] to numerically investigate the
generalizations of the Haldane conjectures to SU(N) spin chains [6,7].
References
Liens :LPMMC |
| MISSING (Weizmann) | Détails Fermer |
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Partial dislocations in higher order topological insulators le vendredi 11 décembre 2020 à 11:00 |
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Résumé : Nonzero weak topological indices are thought to be a necessary condition to bind a single helical mode to a lattice dislocation. I will show that higher-order topological insulators (HOTIs) can, in fact, host a single helical mode along screw or edge dislocations in the absence of weak topological indices. When this occurs, the helical mode is necessarily bound to a dislocation characterized by a fractional Burgers vector, macroscopically detected by the existence of a stacking fault. The robustness of a helical mode on a partial defect is demonstrated by an adiabatic transformation that restores translation symmetry in the stacking fault. Since partial defects and stacking faults are commonplace in bulk crystals, the existence of such helical modes can measurably affect the expected conductivity in these materials. Finally I will describe a general framework towards the classification of symmetry breaking defects based on symmetry representations. Phys. Rev. Lett. 123, 266802 (2019) (arXiv:1809.03518) https://arxiv.org/abs/1908.00011 Liens : |
| Sergeï Andreev (Petersburg Nuclear Physics Institute) | Détails Fermer |
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Pairing of dipolar bosons in semiconductor heterostructures le lundi 07 décembre 2020 à 10:30 |
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Résumé : Pairwise interaction of dipolar excitons has been argued to display the physics of a resonance on a quasi-discrete level. The discrete level (dipolar biexciton) is coupled to the continuum of states (unpaired excitons) by tunnelling through a potential barrier due to the dipolar repulsion. In this talk I will discuss implications of resonant interactions on the quantum collective behaviour of dipolar excitons and their polariton derivatives in 2D semiconductors. Besides dipolar supersolidity and generation of strongly-correlated photons, the dipolar excitons will also be shown to represent an appealing playground for investigation of the bosonic BCS-BEC transition under spin-orbit coupling. Liens : |
| Côme Fontaine (LPMMC) | Détails Fermer |
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(titre non communiqué) le lundi 16 novembre 2020 à 10:30 |
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Liens :LPMMC |
| Oleksandr Marchukov (Technische Universität Darmstadt) | Détails Fermer |
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(titre non communiqué) le jeudi 12 novembre 2020 à 11:00 |
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Liens : |
| MISSING (Institut Néel) | Détails Fermer |
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Dynamics of the non-Hermitian Kitaev chain le lundi 02 novembre 2020 à 10:30 |
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Résumé : We theoretically investigate the non-Hermitian dynamical topology of a driven-dissipative Kitaev
chain. For this, we have employed the third quantization formalism [1] to explore signatures of the
non-trivial topology in the time-evolution of the entanglement spectra. Liens : |
| Riccardo Rossi (Center for Computational Quantum Physics, Flatiron Institute) | Détails Fermer |
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High-order renormalized perturbative approach for strongly-correlated fermions le lundi 19 octobre 2020 à 15:30 |
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Résumé : In this talk I show how perturbation theory can be turned into a viable computational approach for physical systems afflicted by the fermionic sign problem. This is accomplished by designing new numerical approaches to reach arbitrarly-high orders for the bare [1] and the renormalized [2] expansion. I discuss the results that I have obtained for the doped square-lattice Hubbard model in the pseudogap regime, and in frustrated lattices. Finally, I present the first unbiased diagrammatic computation in a broken-symmetry phase by discussing the s-wave superfluid transition in the spin-polarized cubic-lattice attractive Hubbard model. References
Liens :Riccardo RossiCenter for Computational Quantum Physics, Flatiron Institute |
| MISSING | Détails Fermer |
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Dynamics in presence of a non-linear Josephson bath le lundi 12 octobre 2020 à 10:30 |
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Résumé : Quantum open system theory has led to a very efficient description for the Markovian dynamics of small quantum systems weakly coupled to reservoirs at thermal equilibrium: the GKLS (or Lindblad) master equation. The method is well fitted for the case of a linear bath composed of a continuum of harmonic modes, e.g. the electro-magnetic vacuum. However, many complex systems of interest deviate from this picture (e.g. interacting and non-linear baths), and a description of the dynamics they induce on a small quantum system coupled to them is missing. Here we focus on the case of a weakly non-linear chain of Josephson junctions in the regime dominated by the capacitances and investigate its effect on a coupled qubit. We identify the conditions on the parameters of chains such that it induces a Markovian evolution on a small quantum system (e.g. a superconducting qubit or cavity). Surprisingly, we find that the evolution is Markovian at low temperature of the chain and becomes non-Markovian at higher temperature (i.e. the contrary of the case of a harmonic bath). In this high-temperature regime, the bath does not equilibrate fast such that the dynamics of the system cannot be captured by a conventional Lindblad equation. To pass this obstacle, we develop a method inspired from the quantum trajectory paradigm. In our approach, the qubit follows at each instant a Markovian (Lindblad) master equation conditioned on the charge configuration of the chain. The latter is not stable due to the inductive part of the Josephson junctions, but instead can jump at random times towards a neighbor configuration. Our approach brings new tools to investigate the dynamics of complex quantum systems. Liens : |
| Irénée Frérot (ENS-Lyon) | Détails Fermer |
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Probing quantum entanglement in many-body systems via inverse statistical methods of classical physics le mercredi 07 octobre 2020 à 11:00 |
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Résumé : Quantum simulators and computers of intermediate scale, composed of a few tens of individual degrees of freedom, are currently being developed on various experimental platforms (e.g. trapped atoms or ions, superconducting qubits). In these systems, many-body entanglement represents the fundamental resource, allowing them to outperform their classical counterparts for specific tasks, but for the same reason also making their exact simulation by classical machines intractable. Proving that quantum entanglement is indeed present among the individual qubits is therefore a central task for certifying these devices. Such a certification has to rely on statistical correlations measured among the qubits (forming a certain data set), and on controlled assumptions about their internal working. However, today, no scalable method exists for proving that a generic data set is incompatible with a non-entangled state. In this talk, I will present a new approach, inspired by inverse problems of statistical physics and data science, to solve this major problem of quantum information science. Specifically, I will show that this problem is equivalent to building a certain classical model (e.g. an Ising model), whose equilibrium Boltzmann distribution fits the data set of interest -- or showing that no such classical model exists. I will introduce the general concept of entanglement witnesses (Bell's inequalities, EPR-steering criteria...), and show how solving inverse statistical problems leads to optimal, data-tailored entanglement witnesses. Most importantly, this can be achieved in a scalable way under generic assumptions, leveraging on scalable techniques of statistical physics (e.g. Monte-Carlo methods). Liens :Irénée FrérotENS-Lyon |
| Denis Bénâtre (LPMMC) | Détails Fermer |
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Micro-cavity exciton-polaritons in a honeycomb lattice and Berry phase le mercredi 23 septembre 2020 à 11:00 |
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Résumé : Exciton-polaritons in a honeycomb lattice are studied theoretically and numerically. Such a system was chosen for its tunable properties compared to electronic graphene. After explaining briefly the concept of exciton-polaritons, I will review the model without polarization, as done by Nicolas Victorin in his PhD thesis and introduce the Berry phase in that case. Afterwards, I will present the model that takes polarization into account, and acknowledge its effects on the dispersion relation (Dirac cones) and the Berry phase. At the end, I will talk about how the Berry phase can be measured by interferometry. In the future, this study aims to understand how the Berry phase is modified as interactions between polaritons are turned on. Liens :LPMMC |
| Alioscia Hamma | Détails Fermer |
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Title: Quantum complexity, scrambling and black holes le mercredi 16 septembre 2020 à 15:30 |
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Résumé : What makes a quantum evolution complex? Can this complexity be characterized by (entanglement) Entropy? We argue that entropy is not just a number but there is a complexity of entanglement arising from a quantum evolution. Quantum complexity is connected to scrambling and the thermal behavior of black holes. We show that entanglement complexity is connected to the impossibility of realizing a unitary entropy pump (i.e., a freezer). We also study the transitions in complexity due to the doping of a quantum circuit by universal gates and show that the transition to complex entanglement can be obtained by just a single gate. Finally, we present a toy model for the realization of the fast scrambling conjecture as a model for a black hole. Liens : |
| Konstantinos 9 septembre (LPMMC) | Détails Fermer |
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Exploring Kardar-Parisi-Zhang universality sub-classes with exciton polariton Bose-Einstein condensates le mercredi 09 septembre 2020 à 11:00 |
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Résumé : We focus on the statistical properties of the phase of a condensate of one-dimensional exciton polaritons driven by incoherent pumping, whose large-distance and long-time properties belong to the Kardar-Parisi-Zhang (KPZ) universality class, the same which describes out-of- equilibrium interface growth. The spatial phase profile, which is flat without confinement, can be altered by a confining potential such that it acquires a local curvature in the bulk of the sample, thus realizing the analog of a curved interface. By using numerical simulations of the generalized Gross-Pitaevskii equation with and without an external potential, we show that different KPZ universality sub-classes can be accessed. These are associated to the KPZ interface with different geometries, flat or curved, while sharing the same universal scaling properties. In particular, we show that the probability distribution of the condensate phase fluctuations are clearly distinct with and without the external potential and agree with great accuracy with the known theoretical results for KPZ: the Tracy-Widom distributions for the one-point statistics, and correlations of Airy processes for the two-point one in the two cases, although only locally for the curved case. This study promotes the exciton-polariton system as a compelling platform to investigate KPZ universal properties. Liens :LPMMC |
| Marcello Dalmonte (ICTP Trieste) | Détails Fermer |
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Exploring weak- and strong-ergodicity breaking with lattice gauge theories le mercredi 17 juin 2020 à 11:00 |
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Résumé : In this talk, I will present two applications of lattice gauge theories in the study of ergodicity-breaking in low-dimensional statistical mechanics models. The first example concerns the recent observation of anomalously slow dynamics in atom arrays, where ground state atoms are laser-coupled to Rydberg s-states. I will discuss how the constrained dynamics describing such systems is exactly equivalent to the lattice Schwinger model - quantum electrodynamics in one-dimension - in the presence of a topological angle. Beyond serving as demonstration of the first large-scale quantum simulation of a gauge theory, this connection enables an immediate interpretation of the observed dynamics as string inversion. Based on this, I will describe a generic phenomenological framework capturing such slow dynamics basic on simple field theoretical insights, relating this to ‘weak-ergodicity-breaking’, and discuss immediately available extensions in more than one-dimension. The second example concerns instead ergodic-non-ergodic transitions in the context of disorder quantum systems. I will present a numerical analysis of the spectral properties of Abelian lattice gauge theories in one-dimension, and discuss how those are sharply different from spin models without local symmetries (such as the Heisenberg model). In particular, the concomitant effect of Coulomb law and disorder leads to sharp signatures of non-ergodic behavior, depicting a very different scenario from the one observed in 1D and debated in a series of recent works. The talk is based on: arXiv:1902.09551 arXiv:1912.09403 arXiv:2003.11073 Liens : |
| MISSING ( Microsoft Station Q) Annulé | Détails Fermer |
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(titre non communiqué) le mardi 16 juin 2020 à 14:00 |
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Liens : |
| Aniket Rath et Pierrick (LPMMC) | Détails Fermer |
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(titre non communiqué) le mercredi 10 juin 2020 à 11:00 |
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Résumé : Aniket Rath : Liens :LPMMC |
| Thomas Bouillaud et Pierrick (LPMMC) | Détails Fermer |
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Microwave spectroscopy of a dissipative quantum phase transition in Josephson Junctions le mercredi 03 juin 2020 à 11:00 |
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Résumé : Liens :LPMMC |
| Romuald Fournis le (LPMMC) | Détails Fermer |
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QED corrections to the Electromagnetic Abraham force in heavy atoms le jeudi 28 mai 2020 à 11:00 |
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Résumé : Liens :LPMMC |
| Nicolas Bobba et Denis (LPMMC) | Détails Fermer |
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(titre non communiqué) le mercredi 27 mai 2020 à 11:00 |
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Liens :LPMMC |
| Denis Basko (LPMMC) | Détails Fermer |
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Superconductor-insulator transition in Josephson junction chains by Quantum Monte-Carlo le mercredi 13 mai 2020 à 11:00 |
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Résumé : To connect to the conference room, follow the link
https://webconf.math.cnrs.fr/b/ros-wfz-jt4 select "Microphone" and perform the test. That's all.
Liens :LPMMC |
| Ivan Amelio (LPMMC) | Détails Fermer |
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Theory of the coherence of low dimensional and topological lasers le mercredi 29 avril 2020 à 11:00 |
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Résumé : We discuss fundamental aspects of the coherence of large but finite laser arrays in low dimensions (aka non-equilibrium quasi-condenstates). We first focus on 1D chains of laser resonators and study the relation between Kardar-Parisi-Zhang universality and the Schawlow-Townes-like linewidth, which arises as a finite size effect. We then apply these results to investigate the coherence of the edge mode lasing of a 2D Harper-Hofstadter topological laser. Liens :LPMMC |
| Giovanni Pecci (LPMMC) | Détails Fermer |
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Mesoscopic analysis of pairing mechanism in BCS-BEC crossover le mercredi 22 avril 2020 à 11:00 |
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Liens :LPMMC |
| Nicolas Rougerie (LPMMC) | Détails Fermer |
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Wigner crystallization in special space dimensions le mercredi 15 avril 2020 à 11:00 |
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Résumé : One of the most fundamental questions in science is crystallization: the tendency of interacting objects to arrange in periodic structures, e.g. Bravais lattices. Most basically one can consider classical point particles in their ground state, energy minimizing configuration. In this talk we focus on "Wigner crystallization": interactions between particles are purely repulsive and the overall density is fixed. That the ground state is crystalline in the physically relevant space dimensions 2 and 3 is true beyond any reasonable doubt, but a proof or even (to my knowledge) a reasonable theoretical explanation is still missing. I will report on a recent mathematical breakthrough (due to Cohn-Kumar-Miller-Radchenko-Viazovska, NOT to me): a rigorous proof of Wigner crystallization in the special space dimensions 8 and 24. This may sound physically irrelevant but, besides the beautiful mathematics involved, this is as far as I am concerned one of the most convincing theoretical indications backing the reality of crystallization. Liens :LPMMC |
| Bart van Tiggelen (LPMMC) | Détails Fermer |
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Open Access of Publications and Plan S in Europe le mercredi 08 avril 2020 à 11:00 |
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Liens :LPMMC |
| MISSING (Center for Quantum Phenoma, NYU) Annulé | Détails Fermer |
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Phase Signature of Topological Transition in Josephson Junctions le mardi 31 mars 2020 à 14:00 |
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Annulé
Liens : |
| Anastasia Gorbunova (LPMMC) | Détails Fermer |
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Spatio-temporal correlations in turbulence le mercredi 25 mars 2020 à 11:00 |
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Liens :LPMMC |
| Amit Vashisht (LPMMC) | Détails Fermer |
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Polariton-Polariton interaction strength in MoSe2 monolayer le mercredi 18 mars 2020 à 11:00 |
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Liens : |
| MISSING (University of Turku (Finland)) Annulé | Détails Fermer |
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Heat current and energy reactance in a driven quantum system le mardi 17 mars 2020 à 14:00 |
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Annulé
Liens : |
| Jean-Damien Pillet (Laboratoire des Solides Irradiés (Ecole Polytechnique)) | Détails Fermer |
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Guiding Dirac Fermions in Graphene with a Carbon Nanotube le mardi 10 mars 2020 à 14:00 |
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Résumé : Relativistic massless charged particles in a two-dimensional conductor can be guided by a one-dimensional electrostatic potential, in an analogous manner to light guided by an optical fiber. In this seminar, I will present how we use a carbon nanotube to generate such a guiding potential in graphene and create a single mode electronic waveguide. In our architecture, the nanotube and graphene are separated by a few nanometers and can be controlled and measured independently. As we charge the nanotube close to the surface of graphene, we observe in the latter the formation of a single guided mode that we detect using the same nanotube as a probe. I will discuss why the small dimensions of the nanotube and the linear dispersion relation of Dirac fermions gives these electronic waveguides promising characteristics for potential applications. I will also show that, in presence of magnetic field, our electronic waveguides host discrete electronic levels resembling Landau levels of 2D Dirac particles but with no C-symmetric counterpart, i.e. they exist only for one sign of energy, positive or negative, depending on the voltage applied on the nanotube. This unusual behavior is a generic signature of Dirac surface states, which are predicted to be protected to a great extent to surface disorder. Ref: Austin Cheng, Takashi Taniguchi, Kenji Watanabe, Philip Kim, and Jean-Damien Pillet Phys. Rev. Lett. 123, 216804 (2019) Liens : |
| Alessandro Olgiati (LPMMC) | Détails Fermer |
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Stability of the Laughlin phase against long-range interactions le mercredi 19 février 2020 à 11:00 |
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Résumé : The Laughlin wave function is at the basis of the current understanding of the fractional quantum Hall effect, nevertheless, many of its fundamental properties are yet to be understood. I will present a model for its response, within Laughlin's ansatz, to variations of the external potential and of the long-range part of the interparticle interaction. Our main result is that the minimal energy within Laughlin's approximation is asymptotically attained, in the large particle number limit, by states exhibiting uncorrelated quasi-holes on top of the Laughlin wave function. This is a joint project with Nicolas Rougerie. Liens : |
| Steeve Cronenberger (Laboratoire Charles Coulomb (Montpellier)) | Détails Fermer |
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Spatio-temporal spin noise spectroscopy le mardi 18 février 2020 à 14:00 |
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Résumé : In experimental measurements the noise is not necessarily a complication and from it one can extract valuable information regarding its sources. Spin noise spectroscopy (SNS) is a powerful technique to get information about dynamics of various systems (atomic gases, conduction electrons, localized states in semiconductors…) based on their spin fluctuations. As powerful as the technique is, SNS only addresses time spin correlations, but not spatial correlations. Thus valuable informations on spin transport or many-body spin interactions are missed. In this seminar, I shall start with an introduction to SNS from the principles to applications. Then I will present recent advances we made in developing an all-optical probe of spatiotemporal spin fluctuations based on a new SNS approach (1). This new technique gives access to spatial correlations and spin motion. We thus investigated electron spin dynamics in CdTe by measuring simultaneously the electron spin relaxation rate and the spin diffusion constant. In addition, the sensitivity of the SNS allows us to observe the effect of the local nuclear field on the evolution of the electron spin and to estimate the electron spin correlation time on a given donor site. We then face to an unprecedented puzzle we hope to solve. (1) S. Cronenberger, C. Abbas, D. Scalbert and H.Boukari, Phys. Rev. Lett. 123 017401 (2019) Liens : |
| Tanay Nag | Détails Fermer |
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Floquet generation of higher order topological phases and its quenching dynamics le vendredi 14 février 2020 à 11:00 |
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Résumé : Higher order topological (HOT) states, hosting topologically protected modes on lower-dimensional boundaries, such as hinges and corners, have recently extended the realm of the static topological phases. We here demonstrate the possibility of realizing a two-dimensional emph{Floquet} second-order topological insulator, featuring corner localized zero quasienergy modes and characterized by quantized Floquet qudrupolar moment $Q^{Flq}_{xy}=0.5$, by periodically kicking a quantum spin Hall insulator (QSHI) with a discrete four-fold (C4) and time-reversal (T) symmetry breaking mass perturbation. We also demonstrate the generation of a series of higher order topological phase in 3D originated due to Floquet driving with appropriate perturbation. In parallel, we analyze the dynamics of a corner mode after a sudden quench, when the C4 and T symmetry breaking perturbation is switched off, and find that the corresponding survival probability displays periodic appearances of complete, partial and no revival for long time, encoding the signature of corner modes in a QSHI. Our protocol is sufficiently general to explore the territory of dynamical HOT phases in insulators (electrical and thermal) and gapless systems. Liens : |
| Jonathan Wise | Détails Fermer |
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The role of disorder in plasmon-assisted near-field heat transfer between two-dimensional metals le mercredi 12 février 2020 à 11:00 |
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Résumé : We perform a theoretical study of the near-field heat transfer between two atomically thin metallic layers, isolated galvanically but coupled by the Coulomb interaction, within the framework of fluctuational electrodynamics in the Coulomb limit. We clarify the role of disorder and spatial dispersion, and identify several distinct regimes of the heat transfer. We find that the plasmon contribution to the heat current is suppressed both in the clean and diffusive limits, but dominates in a parametrically wide crossover region at sufficiently high temperatures. In the diffusive limit, the heat transfer can be qualitatively modelled by an effective circuit theory. Liens : |
| Cécile Repellin (LPMMC) | Détails Fermer |
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Magnetism and quantum anomalous Hall effect in twisted bilayer graphene le mercredi 05 février 2020 à 11:00 |
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Résumé : Recent experiments have established that moire graphene materials are a platform for several many-body phenomena, including correlated insulators, superconductivity, ferromagnetism, and a (quantized) anomalous Hall effect. In this talk, I will focus on the last two phenomena, which have been observed when conduction and valence bands are nearly flat and are separated by a single-particle gap. This is for example the case in magic angle twisted bilayer graphene (TBG) when one of the graphene layers is aligned with its hexagonal Boron Nitride (hBN) substrate. After explaining the origin of its peculiar band structure, I will turn to the strong interaction regime. I will provide analytical and numerical evidence for spin and valley polarization in these systems at integer filling of the active band. Using the results of exact diagonalization, I will discuss the stability of this 'flat-band ferromagnet' upon increasing the bandwidth. The nearly flat conduction band of TBG aligned with hBN has a Chern number C=1; can it also host a fractional quantum Hall state (without a magnetic field) at fractional filling? Using exact diagonalization results, I will show that these topologically ordered states may indeed emerge in twisted bilayer graphene, albeit with a spin polarization different from what is expected in usual quantum Hall systems. Liens : |
| Vishal Ranjan (Quantronics group, SPEC, Universite Paris-Saclay) | Détails Fermer |
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High sensitivity quantum-limited electron spin resonance le mardi 04 février 2020 à 14:00 |
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Résumé : In a conventional electron spin resonance (ESR) spectrometer based on the inductive detection method, the paramagnetic spins precess in an external magnetic field radiating weak microwave signals into a resonant cavity which are subsequently amplified and measured. Despite its widespread use, ESR spectroscopy has limited sensitivity, and large amounts of spins are necessary to accumulate sufficient signal. Most conventional ESR spectrometers operate at room temperature and employ three-dimensional cavities. At X-band, they require approximately 10^13 spins to obtain sufficient signal in a single echo (1). Enhancing this sensitivity to smaller spin ensembles and eventually the single spin limit is highly desirable. Exploiting recent progress in circuit-quantum electrodynamics, we have combined high quality factor superconducting micro-resonators and noise-less Josephson Parametric Amplifiers to perform ESR spectroscopy at millikelvin temperatures, reaching a new regime where the sensitivity is limited by the quantum fluctuations of the microwave field. Quantum fluctuations of the field also affect directly the spin dynamics via Purcell effect : spin relaxation occurs dominantly by spontaneous emissions of microwave photons. Based on these principles (2-4), we first show an unprecedented measurement sensitivity of about 10 spins per squareroot Hz for unit SNR in an inductive-detection ESR with an ensemble of Bismuth donors in Silicon (5). This high sensitivity enables us to characterize the coherence properties of an ensemble of donors in close proximity (approx. 50 − 100 nm) to the silicon surface, with spatial resolution. We identify surface magnetic and electric noise as the main decoherence sources in our device. At the so-called "clock transition", the coherence time approaches 1s, which is the longest reported for an electron spin close to a surface (6).
(1) A. Schweiger and G. Jeschke, Principles of pulse electron paramagnetic resonance (Oxford University Press, 2001).
Liens : |
| James L. Crowley (Grenoble Institut Polytechnique, Univ. Grenoble-Alps, Chair on Intelligent Collaborative Systems) | Détails Fermer |
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Artificial Intelligence: A Rupture Technology for Scientific Research? le vendredi 31 janvier 2020 à 11:00 |
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Résumé : The Turing test defines intelligence as human-level performance at interaction. After more than 50 years of research, Machine Learning has provided an enabling technology for constructing intelligent systems with abilities at or beyond human level for interaction with people, with systems, and with the world. Artificial Intelligence has emerged as a rupture technology for many areas of engineering science, as well as industrial, societal and commercial innovation. But can AI provide a new tool for modeling and understanding natural phenomena? In this talk I will review of recent progress in Machine Learning, and examine how these technologies change the kind of systems that we can build. Starting with a summary of the multi-layer perceptron and back propagation, I will describe how massive computing power combined with planetary scale data and the world wide web have created the rupture technology known as deep learning. I will discuss common architectures, and review recent advances such as Generative Adversarial Networks, Natural Language Processing and Cognitive Computing. I will describe how these technologies can be used to build systems for collaborative assistance and discuss open problems concerning explainable, verifiable, and trustworthy artificial intelligence. Liens : |
| Anastasia Gorbunova (LPMMC et LEGI) Annulé | Détails Fermer |
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(titre non communiqué) le mercredi 29 janvier 2020 à 11:00 |
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Liens : |
| Carlo Pagani (LPMMC) | Détails Fermer |
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Scalar turbulence, anomalous scaling, and the functional renormalization group le mercredi 22 janvier 2020 à 11:00 |
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Résumé : After briefly reviewing scaling arguments in turbulence, we study the scaling properties of a scalar field advected by a random Gaussian velocity field (the Kraichnan model). We analyze the symmetries of the model and derive its scaling properties in a field theoretic formalism by renormalizing composite operators and by applying the operator product expansion in the framework of the functional renormalization group. Finally, we discuss how to study the spatio-temporal correlation functions within the proposed framework. Liens : |
| Vyacheslavs Kashcheyevs (University of Latvia) | Détails Fermer |
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Continuous-variable tomography of on-demand ballistic electrons le mardi 21 janvier 2020 à 14:00 |
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Résumé : Individual electrons in semiconductors circuits that can either be confined to quantum dots or guided ballistically along nanowire edges form an exciting quantum technology platform being actively developed for applications in metrology, quantum sensing and quantum information transfer. Precision of electron counting is rivalling most precise conventional measurements of small currents in ongoing progress towards direct realisation of the quantum ampere in the redefined SI. Availability of on-demand electron sources has enabled chip-level collider and tomography experiments aimed to develop a full toolkit for a few-electron quantum optics. We discuss a recently proposed and implemented quantum tomography technique for solitary electrons propagating in a chiral edge state in a depleted region of a two-dimensional quantum Hall system. Quasi-probability distribution of complementary continuous variables — arrival time and energy — is accessed by measuring probabilities of scattering from a time-dependent energetically sharp gate-controlled barrier. Wigner distribution is recovered from the spectrum of projections at different modulation speeds using standard techniques of tomographic imaging. We resolve a chirp in the energy of electrons emitted on demand from a tuneable barrier pump, and put a lower bound on the quantum coherence of the source. Liens : |
| Floris Braakman (University of Basel) | Détails Fermer |
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Ultrafast and electrically tunable coherent operations of hole spin qubits le mardi 14 janvier 2020 à 14:00 |
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Résumé : The spin state of single holes in Ge- and Si-based one-dimensional nanostructures can be used to encode a qubit with unique functionality. Such hole spin qubits potentially combine all-electrical control, ultrahigh clock speeds and small device sizes, promising a level of control that goes beyond that of conventional electron spin qubits. In particular, the spin-orbit interaction of holes in Ge/Si core-shell nanowires is predicted to be both very strong and electric field tunable. Such electrical tunability would enable to switch spin-orbit interaction either on, enabling fast quantum operations, or off, leading to improved coherence times. In recent experiments, we have demonstrated the presence of this strong spin-orbit interaction and have used it to show record spin qubit Rabi frequencies exceeding 400 MHz and entering the regime of strong driving. Furthermore, we find the Rabi oscillation frequency as well as the g-factor to be highly tunable through small changes in gate voltages, indicating electrical control over the spin-orbit interaction strength. These measurements demonstrate the viability of hole spin qubits in one-dimensional Ge- and Si-based nanodevices as a platform for the implementation of fast and scalable quantum computation. Liens : |
| Oleksandr Marchukov | Détails Fermer |
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Quantum fluctuations and uncertainty relations in NLS solitons and breathers le mercredi 08 janvier 2020 à 11:00 |
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Résumé : We consider the quantum fluctuations of the macroscopic variables associated with a breather, a second-order soliton solution of nonlinear Schrödinger equation. Linearizing the evolution of the bosonic quantum field around the Bose condensate in a breather state, we express the quantum fluctuations of the macroscopic variables through the fluctuations of the full quantum field. We compare two models for the state of the quantum field of fluctuations surrounding the classical field of the Bose-Einstein condensate: a conventionally used "white noise" and a correlated noise which assumes that the breather has been created from a fundamental soliton, by means of the application of the factor-of-four quench of the nonlinearity strength. We evaluate the initial quantum uncertainties of the macroscopic parameters and their time evolution. This approach is well suited for the description of Bose gas with large number of atoms and suggests the possibility for experimental observation of macroscopic quantum fluctuations. Liens : |
| Sergio Ciliberto (ENS Lyon) | Détails Fermer |
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A protocol for reaching equilibrium arbitrary fast le mardi 07 janvier 2020 à 14:00 |
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Résumé : When a control parameter of a system is suddenly changed, the accessible phase space changes too and the system needs its characteristic relaxation time to reach the final equilibrium distribution. An important and relevant question is whether it is possible to travel from an equilibrium state to another in an arbitrary time, much shorter than the natural relaxation time. Such strategies are reminiscent of those worked out in the recent field of Shortcut to Adiabaticity, that aim at developing protocols, both in quantum and in classical regimes, allowing the system to move as fast as possible from one equilibrium position to a new one, provided that there exist an adiabatic transformation relating the two. Proof of principle experiments have been carried out for isolated systems. Instead in open system the reduction of the relaxation time, which is frequently desired and necessary, is often obtained by complex feedback processes. In this talk, we present a protocol, named Engineered Swift Equilibration (ESE), that shortcuts time-consuming relaxations, We tested experimentally this protocol on a Brownian particle trapped in an optical potential first and then on an AFM cantilever. We show that applying a specific driving, one can reach equilibrium in an arbitrary short time. We also estimate the energetic cost to get such a time reduction. The ESE method paves the way for applications in micro and nano devices, in high speed AFM, or in monitoring mesoscopic chemical or biological process.
References: Liens : |
| Loic Herviou (KTH Stockholm) | Détails Fermer |
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Bulk-edge correspondence for non-Hermitian Hamiltonians:singular-value decomposition and entanglement spectrum le mardi 07 janvier 2020 à 11:00 |
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Résumé : Non-Hermitian systems are known to present a breakdown of the traditional bulk-boundary correspondence, with open and periodic systems that can have distinct phase diagrams. The correspondence can be completely restored by considering the Hamiltonian's singular value decomposition instead of its eigendecomposition. This leads to a natural topological description in terms of a flattened singular decomposition. This description is equivalent to the usual approach for Hermitian systems and coincides with a recent proposal for the classification of non-Hermitian systems. We also propose several generalizations of the notion of the entanglement spectrum to non-Hermitian systems, and show how it can capture either capture the true edge physics or (partially) verify standard bulk-edge correspondence. Liens : |
| Philippe Jacquod (HES-SO (Haute Ecole Spécialisée de Suisse Occidentale), Suisse) | Détails Fermer |
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The key player problem in complex oscillator networks and electric power grids le vendredi 20 décembre 2019 à 11:00 |
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Résumé : In network theory, two issues of central importance are (i) how to assess the global robustness of a network-coupled system and (ii) how to identify its local vulnerabilities. The second issue is related to the historical and fundamental problem of identifying the key player. That may be for instance the player who, once removed, leads to the biggest changes in the other player’s activity in game theory, or to the biggest structural change in a social network. That problem has been tackled with the introduction of graph-theoretic descriptors, in particular centrality indices. Additionally, centralities averaged over the whole system provide a global indicator of how tightly bound a network is, which helps in dealing with the first issue. The purely graph theoretic, centrality-based approach cannot be straightforwardly applied to deterministic network-coupled dynamical systems. Assessing such a network’s global robustness and identifying its most critical components must go beyond computing graph centralities and needs to incorporate the coupling dynamics into account. In my talk I will discuss methods recently developed to deal with these two issues in physical network-coupled dynamical systems. The talk will survey a number of already obtained results and present a tentative discussion of future challenges. Reference: M. Tyloo, L. Pagnier, P. Jacquod, "The key player problem in complex oscillator networks and electric power grids: Resistance centralities identify local vulnerabilities", Science Advances 5, eaaw8359 (2019) Liens :Philippe Jacquod |
| Natasha Perkins (University of Minnesota) Annulé | Détails Fermer |
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The pursuit of fractionalized excitations in Kitaev Materials le jeudi 19 décembre 2019 à 11:00 |
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Annulé
Liens :Natasha Perkins |
| Maxim Kharitonov (University of Würzburg) | Détails Fermer |
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Universality and stability of the edge states of chiral-symmetric topological semimetals and surface states of the Luttinger semimetal le vendredi 13 décembre 2019 à 11:00 |
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Résumé : We theoretically demonstrate that the chiral structure of the nodes of nodal semimetals is responsible for the existence and universal local properties of the edge states in the vicinity of the nodes. We perform a general analysis of the edge states for an isolated node of a 2D semimetal, protected by chiral symmetry and characterized by the topological winding number N. We derive the asymptotic chiral-symmetric boundary conditions and find that there are N et 1 universal discrete classes of them. The class determines the numbers of flat-band edge states on either side off the node in the 1D spectrum and the winding number N gives the total number of edge states. We then show that the edge states of chiral nodal semimetals are robust: they persist in a finite-size stability region of parameters of chiral-asymmetric terms. This significantly extends the notion of 2D and 3D topological nodal semimetals. We demonstrate that the Luttinger model with a quadratic-node for j=3/2 electrons (Luttinger semimetal) is a 3D topological semimetal in this new sense and predict that alpha-Sn, HgTe, possibly Pr2Ir2O7, and many other semimetals described by it are topological and exhibit surface state. Reference: M. Kharitonov, J.-B. Mayer, and E. M. Hankiewicz, Phys. Rev. Lett. 119, 266402 (2017). Liens : |
| Daniel Hernangómez Pérez (University of Regensburg) | Détails Fermer |
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Aspects of topology in organic quantum wires le vendredi 06 décembre 2019 à 11:00 |
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Résumé : In the past forty years, polyacetylene molecular wires have attracted a long-standing interest: these wires support propagation of topological domain-wall states, so-called solitons, which provide a paradigm for spin-charge separation. Recent experimental developments have shown that individual polyacetylene chains can be synthesized on metallic substrates. Motivated by this breakthrough, we propose a way for chemically supported “soliton-design†in these systems. We demonstrate how to control the soliton position and how to read-it out by electrical means. This provides a step toward functional electronic devices based on soliton manipulation, i.e. solitonics. |
| Robert Withney (LPMMC Grenoble) | Détails Fermer |
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A non-equilibrium system as a demon le vendredi 29 novembre 2019 à 11:00 |
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Résumé : A Maxwell demon is a creature (or machine) that reduces the entropy of a system without performing any work on it. It performs this apparent violation the second-law of thermodynamics through the intricate action of measuring individual particles and subsequently performing feedback. Bennett (building on work of Landauer) argued that the second-law is restored once one takes into account the fact that information recorded by the demon is a physical resource like heat or work. Here we show that much simpler setups can also act as demons: we demonstrate that it is sufficient to exploit a non-equilibrium distribution to seemingly break the second law of thermodynamics. No particle-by-particle measurement or feedback is necessary. We call this an N-demon (with the "N" for non-equilibrium), and show that it can reduce the entropy of a system without doing work or exchanging heat with that system. We then show that the second-law is restored by treating ``non-equilibrium'' as a physical resource like heat, work or information. We propose both an electronic and an optical implementation of this phenomenon, realizable with current technology. These examples make it clear that the non-equilibrium distribution can be classical or quantum in nature. Ref: Rafael Sánchez, Janine Splettstoesser, Robert S. Whitney, to appear in Phys. Rev. Lett. Eprint - arXiv:1811.02453 Liens : |
| Benoit Vermersch (LPMMC, Grenoble) | Détails Fermer |
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Probing and verifying quantum simulators and quantum computers with randomized measurements le mardi 26 novembre 2019 à 14:00 |
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Résumé : Randomized measurements have emerged as a new tool to probe quantum simulators and quantum computers beyond standard observables. In this talk, I will describe the framework of our randomized measurement protocols that can measure entanglement, out-of-time-ordered correlations, and many-body topological invariants. I will also show some experimental results (Collaboration with the group of Rainer Blatt, IQOQI Innsbruck). Liens : |
| Sergej Moroz (TU Munich) | Détails Fermer |
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Confined phases of fermions coupled to Z2 gauge fields le vendredi 22 novembre 2019 à 11:00 |
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Résumé : After briefly summarizing my long-term interest in quantum physics of low-dimensional spinless fermions that attract each other, I will present our recent study of a quantum many-body lattice system of one-dimensional fermions interacting with a dynamical Z2 gauge field. The gauge field mediates long-range attraction between fermions resulting in their confinement into bosonic dimers. At strong coupling we developed an exactly solvable effective theory of such dimers with emergent constraints. I will show that even at a generic coupling and fermion density, the model can be rewritten as a local spin 1/2 chain and forms a Luttinger liquid. In a finite chain we observed the doubling of the period of Friedel oscillations which paves the way towards experimental detection of confinement in this system. Finally, I will also discuss the possibility of a Mott phase at the commensurate filling 2/3, connection to quantum scars and our plans to extend this study to two spatial dimensions in pursuit of exotic p-wave superfluidity. Liens :Sergej Moroz |
| Théotime Girardot (LPMMC) | Détails Fermer |
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Average field approximation for almost bosonic anyons in a magnetic field le mercredi 20 novembre 2019 à 11:00 |
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Liens :LPMMC |
| Pierre Nataf (LPMMC, Grenoble) | Détails Fermer |
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Superradiant Quantum Phase transition in Rashba Cavity QED le mardi 19 novembre 2019 à 14:00 |
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Résumé : In cavity quantum electrodynamics (QED), the interaction between an atomic transition and the cavity field is measured by the vacuum Rabi frequency omega_0. The regime with omega_0 comparable to the two-level transition frequency is called the ultrastrong coupling regime. In such a regime, and for a large number of atoms coupled to the same cavity mode, a superradiant quantum phase transitions (SQPT) has been predicted, e.g. within the Dicke model. In this theoretical seminar, I will briefly describe the SQPT at equilibrium, discuss the experimental context, and present our recent proposal where a 2DEG with Rashba spin-orbit coupling placed inside an optical cavity can exhibit the SQPT. Ref: P. Nataf, T. Champel, G. Blatter, and D. M. Basko, Rashba cavity qed: a route towards the superradiant quantum phase transition, arXiv:1907.02938 (2019) Liens : |
| CPTGA 15 novembre (Café (Institute of Physics of the Polish Academy of Sciences) | Détails Fermer |
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Properties of open quantum graphs and microwave networks le vendredi 15 novembre 2019 à 11:00 |
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Résumé : We will discuss interesting properties of open quantum graphs and microwave
networks [1]. We will demonstrate that there exist graphs which do not obey the Weyl’s law
N(R) = LR/Ï€ et O(1), where O(1) is a function which for R going to infinity is bounded by a
constant. The Weyl’s law directly links the counting function N(R) of the number of
resonances with the square root of energy k, 0 Liens : |
| Olivier Coquand (German Aerospace Center, Cologne) | Détails Fermer |
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Dynamics of granular fluids le mercredi 06 novembre 2019 à 11:00 |
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Résumé : Granular fluids are omnipresent in our everyday life, from the physics of geological phenomena to food processing. However, very few theoretical models are able to provide reliable predictions for these systems in density ranges close to those encountered on Earth. In this seminar, I will discuss how the combination of the mode-coupling theory, and the integration through transients formalism can be used to describe granular flows at moderate densities (close to the transition to the amorphous solid state). On the particular example of a planar shear flow, the results of this model will be compared to the phenomenological laws of the so-called mu(I) rheology known from experiments Liens : |
| Srijit Goswami (TUDelft) | Détails Fermer |
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Developing InSb quantum wells as a platform for topological superconductivity le mardi 05 novembre 2019 à 14:00 |
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Liens : |
| thèse de Nicolas Victorin (LPMMC) | Détails Fermer |
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Multi-component Gauge Dependent Quantum Gases le vendredi 18 octobre 2019 à 10:00 |
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Liens : |
| Riccardo Rossi (Flatiron Insitute, Smons Foundation, New York) | Détails Fermer |
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New Routes Up the Strongly-Correlated Mountain le jeudi 17 octobre 2019 à 11:00 |
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Liens : |
| Gerbold Menard (SPEC, CEA, Université Paris Saclay) | Détails Fermer |
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Two-terminal conductance measurements in Majoranas SAG nanowires le mardi 15 octobre 2019 à 14:00 |
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Résumé : Majorana quasiparticles have generated years of intense research following the first observation of zero-bias anomalies in semiconductor-superconductor heterostructures [1] due to the promises they hold in the field of quantum computing. However, despite these efforts, definitve proofs of the topological nature of these excitations are still being sought after. In particular, one of the most significative prediction for Majorana fermions is that the zero-bias anomalies are to be found at both sides of a wire. Performing a simultaneous conductance measurements at both sides of a wire would be a significative step forward allowing us to verify this theoretical prediction. Unfortunately, usual InAs nanowires are grown on a substrate before being transferred onto another chip before being processed, which prevents from defining a well-defined electrical ground without defects in the center of the wire. An alternative to these standing wires are the so-called SAG [2] (selective area growth) wires that grow directly on a chip that can be directly processed and can be connected electronically through the epitaxial Al thin film deposited in MBE. Using this technique, we realized three-terminals nanowire structures allowing us to probe both sides of the same wire simultaneously [3,4]. In this presentation, I will discuss the advantage of this SAG wires and present results we obtained in these systems in relation to topological signatures as well as future possible developments using this technique.
[1] V. Mourik et al., Science 336, 1003 (2012)
Liens : |
| thèse de Davide Squizzato (LPMMC) | Détails Fermer |
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Exploring Kardar-Parisi-Zhang universality class: from the dynamics of exciton-polariton condensates to stochastic interface growth with temporally correlated noise le mercredi 09 octobre 2019 à 14:00 |
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Liens : |
| phase slips in one-dimensional superconductors (LPMMC) | Détails Fermer |
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Quantum phase slips in one-dimensional superconductors le mercredi 02 octobre 2019 à 14:00 |
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Liens : |
| MISSING (Université de Rochester) | Détails Fermer |
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Thermodynamics in presence of quantum measurements le mercredi 25 septembre 2019 à 11:00 |
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Résumé : Much progress has been done recently in identifying the consequences of quantum mechanics on thermodynamics. One of the key differences with classical systems is the disturbance induced by quantum measurement on the measured object. This effect is accompanied with energy and entropy changes of the measured system. Consequently, quantum measurement can be considered a thermodynamic resource, and can be used e.g. to fuel quantum engines or refrigerators with no classical analogues. Such engines have different constraints than thermal engines, and can e.g. reach unit efficiency at non-zero power in certain limits. Due to the intimate link between decoherence and measurement, this approach gives new tracks to understand the thermodynamic constraints on quantum protocols, such that quantum computation algorithms. Liens :Université de Rochester |
| Cyril Elouard (University of Rochester) | Détails Fermer |
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Quantum signatures in the heat flow: the case of the fluorescence of a driven two-level atom le mardi 24 septembre 2019 à 14:00 |
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Résumé : A lot of attention was devoted recently to understand the differences between the thermodynamic behavior of classical and quantum systems, and design engines exploiting these differences. One fundamental difficulty to characterize energy transfers in the quantum world is the existence of coherent superpositions of state of different energies. The impact of this property can be investigated by looking at the minimal example of a two-level atom driven quasi-resonantly and in contact with a thermal bath. The driving continuously induces coherences in the energy eigenbasis of the free qubit, which is also the eigenbasis of the state realizing thermal equilibrium with the bath. Consequently, the atom reaches a steady-state which is out-of-equilibrium in a genuinely quantum way. The continuous decoherence induced by the bath is associated with a quantum contribution to the total heat flow provided by the bath which can be interpreted as the energy cost to erase coherences. Identifying allows to derive a quantum version of the first and second law taking into account the presence of coherences, and allowing to study important unsolved problems like the characterization of the energy transfers occuring during a quantum measurement, or the evaluation of the cost required to operate a quantum computer. Liens : |
| Pierre-Olivier Guimond (IQOQI, Innsbruck) | Détails Fermer |
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Chiral quantum optics with atomic arrays and superconducting circuits le jeudi 19 septembre 2019 à 11:00 |
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Résumé : As the size and complexity of quantum processors increase, the ability to route quantum information between distant components in a reliable and controllable way becomes a necessity. In most experiments with superconducting circuits, this task is taken care of by ferrite junction circulators; however, as these devices are bulky and use large magnetic fields, they are not suitable for on-chip integration and new, scalable alternative must be developed in the near future. In this talk I will present the design of a passive integrated architecture for realizing on-chip photon routing. In contrast to other recent proposals, our scheme does not rely on breaking time-reversal symmetry; rather, the collective emission of pairs of superconducting artificial atoms in a microwave transmission line is engineered such that orthogonal atomic transitions spontaneously emit and absorb photons propagating in opposite directions. I will show how the resulting cascaded interactions between distant atoms can be exploited to passively probe and measure programmable many-body operators, which will be illustrated with the generation and manipulation of the toric code. Finally, I will discuss how some of these results can be translated to the optical domain for cold atom experiments, and, in particular, show that photon routing can be realized in free-space with defect-free atomic arrays. |
| MISSING (LPMMC) | Détails Fermer |
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Un niveau quantique discret fortement couplé à un continuum avec une structure de bandes le mercredi 18 septembre 2019 à 14:30 |
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Liens : |
| Gernot Schaller (TU Berlin) | Détails Fermer |
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A strong-coupling approach to electronic quantum transport le mardi 17 septembre 2019 à 14:00 |
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Résumé : The reaction-coordinate mapping is a way of redefining the boundary between system and reservoir that allows to treat limits inaccessible with standard approaches. It is implemented by identifying collective reservoir degrees of freedom and including them -- at the level of the Hamiltonian -- into a redefined system. In particular regimes, this enlarged system can then be treated with standard methods. Within the context of electronic quantum transport, it is straightforward to apply a fermionic version of the mapping individually to every reservoir. This allows to revisit nonequilibrium phenomena from the perspective of strong-system reservoir couplings and non-Markovian effects. In particular, I will demonstrate the benefits of the method by showing that performance of a continuously operating quantum heat engine may increase in the strong-coupling regime. Furthermore, for explicit feedback loops, the method can also be used to identify the thermodynamic cost of measurement and control operations, which for example allows for a revisiting of electronic Maxwell demons. Even models that are anyways exactly solvable may profit from conceptual insight gained from such transformations, which e.g. allows to identify non-Markovian limits by broken thermodynamic uncertainty relations.
Papers: Liens : |
| Tomas Ramos (Characterizing photon-photon interactions and correlated noise in nanophotonic systems) | Détails Fermer |
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Characterizing photon-photon interactions and correlated noise in nanophotonic systems le vendredi 13 septembre 2019 à 11:00 |
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Liens : |
| Eric Woillez (Technion) | Détails Fermer |
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Is the Solar System stable? le jeudi 12 septembre 2019 à 11:00 |
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Résumé : The search for the Solar System's stability is a fascinating adventure that started with Newton's discovery of the law of gravitation. While this question has been the source of major discoveries both in mathematics and in physics for about three centuries, it is still not fully answered. In the past 30 years, a breakthrough occurred with the numerical discovery that the Solar System is chaotic with a Lyapunov time of about 10 million years. In particular, it has been shown that planetary collisions are possible between the four smallest terrestrial planets. Chaotic motion thus prevent any long-term accurate prediction of planetary positions, and requires us to invent new techniques to predict the state of the Solar System on a timescale comparable to its lifetime. In the present talk, I will show how the methods issued from statistical physics can be used to study the long-term stability of the Solar System. I will explain how the probability of fast destabilizations can be predicted using the theory of rare events. Liens : |
| Davide Squizzato (LPMMC) | Détails Fermer |
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Kardar-Parisi-Zhang Equation with temporally correlated noise: a non-perturbative renormalization group approach le mercredi 11 septembre 2019 à 11:00 |
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Liens :Davide SquizzatoLPMMC |
| Freek Massee (LPS (Paris-Sud)) | Détails Fermer |
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Detection and manipulation of dopants and atoms in a high-Tc superconductor using MHz current noise le mardi 10 septembre 2019 à 14:00 |
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Résumé : Dopants and impurities are crucial in shaping the ground-state of host materials: semiconducting technology is based on their ability to donate or trap electrons, and they can even be used to transform insulators into high temperature superconductors. Due to limited time resolution, most atomic scale studies of the latter materials focussed on the effect of dopants on the electronic properties averaged over time. To lift this limitation, I will first present how we implemented cryogenic circuitry operating in the MHz regime into our home-built scanning tunnelling microscope in order to gain access to time-dependent information, including shot-noise, at the atomic scale [1]. After discussing the details of the circuitry, I will show how it enabled us to detect remarkable charge dynamics at select atomic sites in the high temperature superconductor Bi2Sr2CaCu2O8 et x [2]. Lastly, I will demonstrate how we can use these sites, as well as other individual atoms, to manipulate superconductivity.
[1] F. Massee et al., Rev. Sci. Instrum. 89, 093708 (2018) Liens : |
| Amit Ghosal (IISER Calcutta) | Détails Fermer |
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Superconductivity in a disordered vortex lattice le lundi 09 septembre 2019 à 11:00 |
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Résumé : Orbital magnetic field and strong disorder weaken superconducting correlations acting individually on a type-II s-wave superconductor. The Abrikosov vortex lattice, resulting from the applied magnetic field, melts with an increase of the strength of the field, turning the system into a metal. Similarly, the presence of disorder causes a superconductor to insulator transition beyond a critical strength of disorder. Here we show that the interplay of these two perturbations, when present simultaneously in a two-dimensional superconductor, causes its intriguing evolution. In particular, we show that the local superconductivity can actually strengthen due to interesting spatial reorganization or order parameters in the presence of strong disorder. While at weak disorder strengths the critical magnetic field for the suppression of superconducting energy gap matches with the critical field at which superfluid density vanishes, the two critical fields diverge from each other with the increase of the disorder strengths. Our results have important consequences for the strong magnetoresistance peak observed in disordered superconducting thin films. We illustrate this by calculating the dynamical conductivity and analyzing its low-frequency behavior. Our results, which emphasize the role of spatial fluctuations in the pairing amplitude, capture the non-monotonic evolution of the magnetoresistance, consistent with experiments. We will also demonstrate that the presence of even weak disorder causes the Caroli-deGennes-Matricon zero-bias peak in vortex-core density of states to disappear. The origin and consequences of such dramatic behaviors will be discussed along with their experimental relevance. * Work done in collaboration with Anushree Datta, Anurag Banerjee, and Nandini Trivedi Liens :Amit Ghosal |
| Eli Levenson-Falk (University of Southern California) | Détails Fermer |
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Harnessing Noise in Superconducting Quantum Circuits le mardi 03 septembre 2019 à 14:00 |
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Résumé : Superconducting quantum circuits show great potential as a practical quantum information technology. However, noise causes decoherence and loss of fidelity in quantum processes, preventing full-scale quantum processors from being built. I will discuss our ongoing experiments to harness noise to improve coherence and fidelity. These include adding engineering the quantum bath to turn bad qubits into a good refrigerators; adding "generalized Markovian" noise to suppress the effects of environmental Markovian noise; and using noise correlations between different qubits to design better quantum error correction algorithms. Liens : |
| Uwe Tauber (Virginia Tech) | Détails Fermer |
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Nucleation and Aging Transient Dynamics in the Two-Dimensional Complex Ginzburg-Landau Equation le mercredi 28 août 2019 à 11:00 |
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Résumé : The complex Ginzburg-Landau equation (CGL) is a (stochastic)
partial differential equation that describes a remarkably wide
range of physical systems. We numerically investigate nucleation
processes in the transient dynamics of the two-dimensional CGL
towards its "frozen" state with stationary spiral structures,
starting either from the defect turbulence regime or random
initial configurations. Nucleation events of spiral structures
are monitored using the characteristic length between the
emerging shock fronts. We employ an extrapolation method and a
phenomenological formula to account for finite-size effects. The
non-zero barrier for the nucleation of single spiral droplets in
the extrapolated infinite system size limit suggests that the
transition to the frozen state is discontinuous. We also study
the nucleation of spirals for systems that are quenched close to
but beyond the crossover limit, and of target waves which emerge
if a specific spatial inhomogeneity is introduced. In either of
these cases, we observe long, "fat" tails in the distribution of
nucleation times, which also supports a discontinuous transition
scenario. Upon quenching the CGL into the "defocusing spiral
quadrant", we observe slow coarsening dynamics as oppositely
charged topological defects annihilate. We find the physical
aging features in this system to be governed by non-universal
aging scaling exponents. We also investigate systems with control
parameters residing in the "focusing quadrant", and identify slow
aging kinetics in that regime as well. We provide heuristic
criteria for the existence of slow coarsening dynamics and
physical aging behavior in the CGL.
Liens : |
| David Rodriguez Fernandez | Détails Fermer |
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Hall viscosity induced transverse voltage in two-dimensional Fermi liquids le vendredi 19 juillet 2019 à 11:00 |
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Résumé : The absence of parity and time-reversal symmetry in two-dimensional Fermi liquids gives rise to nondissipative transport features characterized by the Hall viscosity. For non-vanishing magnetic fields, the Hall viscous force directly competes with the Lorentz force, since both mechanisms contribute to the Hall voltage. In this work, we present a channel geometry that allows us to uniquely distinguish these two contributions and derive, for the first time, their functional dependence on all external parameters. In particular, the ratio of Hall viscous to Lorentz force contributions decreases with the width and slip-length of our channel, while it increases with its carrier density and electron-electron mean free path. Therefore, for typical materials such as GaAs, the Hall viscous contribution can dominate the Lorentz signal by orders of magnitudes up to a few tens of millitesla. This paves the way to uniquely measure and identify Hall viscous signals in simple experimental setups. Liens : |
| Alioscia Hamma (UMass Boston) | Détails Fermer |
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Quantum complexity, irreversibility, learnability and fluctuations le vendredi 12 juillet 2019 à 11:00 |
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Résumé : Quantum complexity is a notion characterizing the universality of the entanglement arising from a quantum evolution. A universal evolution will result in a complex entanglement. At the same time, this also corresponds to small fluctuations and to unlearnability from the point of view of machine learning. All these aspects are connected to the different features of k-designs, which are under-samplings of the Hilbert space. We study the transition in complexity due to the doping of a quantum circuit by universal gates and show that the transition to complex entanglement can be obtained by just a single gate. These results are relevant for the notions of scrambling, quantum chaos, OTOCs and operator spreading. We conjecture that the transition to 4−design, W-D and unlearnability are one and the same. Liens : |
| MISSING (University of Southern California) Annulé | Détails Fermer |
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Quantum coherence in the localization transition le mercredi 10 juillet 2019 à 11:00 |
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Annulé
Liens : |
| Pascal Simon (LPS - Orsay) | Détails Fermer |
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Majorana zero modes around skyrmionic textures' le vendredi 05 juillet 2019 à 11:00 |
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Résumé : Recent scanning tunneling spectroscopy measurements on a superconducting monolayer of lead(Pb) with nanoscale cobalt islands, have revealed puzzling quasiparticle in-gap states [1] which demand a better understanding of two-dimensional superconductivity in presence of spin-orbit coupling and magnetism. Thus motivated, we theoretically study a model of two-dimensional s-wave superconductor with a fixed configuration of exchange field and spin-orbit coupling terms allowed by symmetry. Using analytics and exact diagonalization of tight-binding models, we find that a vortex-like defect in the Rashba spin-orbit coupling binds a single Majorana zero-energy (mid-gap) state. In contrast to the case of a superconducting vortex [2], our spin-orbit defect does not create a tower of in-gap excitation states and our findings match the puzzling features observed in the experiment. Additionally, these properties indicate that the system realizes a pair of well-protected Majorana zero mode (MZM) localized at the core and the rim of the defect [3]. We also discuss how the quasiparticle states of the defect relate to the states of superconductors on top of magnetic textures, such as skyrmions. Magnetic skyrmions are nanoscale particle-like spin configurations that are efficiently created and manipulated. They hold great promises for next-generation spintronics applications. I will focus on the theoretical analysis of magnetic skyrmions proximitized by conventional superconductors. I will show that a topological superconducting phase can emerge in these systems and uncover a whole almost flat band of these modes on the edge of the magnetic texture, in contrast to a previously reported MZM in the core of the skyrmion [4]. I will discuss in details the origin of these MZMs by relating this problem to the the extensively-studied Rashba nanowire model. We have found that these modes are remarkably stable to electronic and geometric perturbations which we investigate by a combination of analytical arguments and numerical tight-binding calculations. Additionally, this analysis reveals that the number of MZMs on the edge scales linearly with its perimeter [5]. [1] G.C. Ménard et al., Nature Comm. 11, 1013 (2017). [2] C. Caroli, P.G. de Gennes, and J. Matricon, Physics Letters 9, 307(1964). [3] G. C. Ménard, et al., arXiv:1810.09541, Nature Comm. 10, 2587 (2019). [4] G. Yang, P. Stano, J. Klinovaja & D. Loss, PRB 93, 224505 (2016). [5] M. Garnier, A. Mesaros, P. Simon, arXiv:1904.03005 Liens : |
| Antonio Rago (University of Plymouth) | Détails Fermer |
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Multi-particle observables from pure Yang Mills le jeudi 04 juillet 2019 à 11:00 |
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Résumé : The strong force is governed by a mathematical framework called quantum chromodynamics (QCD). The building blocks of QCD are quarks and gluons, and the interactions of these constituents leads to a rich variety of observed phenomena. A particularly intriguing aspect of QCD physics is the nature and behavior of resonances, short-lived states that decay via the strong force. In this talk I will discuss progress in studying these states, by calculating multi-particle scattering observables in a the simplified framework of pure Yang Mills. This can be achieved by combining field-theoretic ideas with large scale numerical calculations. In particular, I will focus on the idea of using the finite volume required for numerical calculations as a tool, rather than an unwanted artifact, to extract dynamical observables such as two- and (eventually) three-particles scattering amplitudes. Liens : |
| Anastasia et Bastien (LPMMC) | Détails Fermer |
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(titre non communiqué) le mercredi 03 juillet 2019 à 11:00 |
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Résumé : Anastasia Gorbunova : Numerical study of turbulence Bastien Maguet : Symmetries in the stochastic dynamics of interfaces and their supersymmetric formulation Liens : |
| Stefano Roddaro (Scuola Normale Superiore & UniversitaÌ€ di Pisa) | Détails Fermer |
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Field-effect control of the properties of InAs/InP nanowire single-electron trnasistors le mardi 02 juillet 2019 à 14:00 |
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Résumé : Single-electron transistors based on heterostructured nanowires represent a promising and robust building block for a range of applications in fundamental science as well as in sensing. Many of these require a good degree of control on a set of key device parameters such as the tunnel coupling, level spacing and filling, which is not obvious to combine with the adoption of a heterostructure-defined nanodevice. In my talk I will review our recent results on the field-effect control of InAs/InP quantum dots and demonstrate in particular how tunnel rates can be sharply and controllably increased depending on the kind of orbitals involved in the transport process. Experimental results will be compared with simulations of the nanostructure to identify the mechanisms responsible for the tuning. Liens : |
| Maxim Olchanyi (LPMMC) | Détails Fermer |
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Lax Integrability and Cheap Macroscopic Quantum Coherence with Matter-Wave Breathers le lundi 1er juillet 2019 à 10:00 |
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Résumé : Matter-wave breathers is an example of an area where the macroscopic quantum coherence resource may be already available today but yet overlooked. There, even for a relatively hot soliton, a four-fold quench of the coupling constant will generate a bi-solton state whose relative soliton-soliton motion is in a minimal Heisenberg uncertainty state. The latter will be observable through an eventual separation between the solitons, itself a deep consequence of the Lax integrability and the classical field level and Bethe integrability at the quantum one. The estimates for the separation time range between a few to a dozen of seconds, i.e. within the experimental reach. Liens :LPMMC |
| Maxim Olchanyi (UMass Boston) | Détails Fermer |
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Some Empirical Implementations of the Multi-Dimensional Reflection Groups le vendredi 28 juin 2019 à 11:00 |
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Résumé : In this presentation, I will review some of our recent successes in finding a three-dimensional empirical room for the abstract multidimensional kaleidoscopes. The latter ensure solvability of the former. The areas of implementation include (a) quantum one-dimensional hard-core particles with mon-trivial mass-spectra, on a line, in a box, or in a harmonic potential; (b) a quantum one-dimensional bosonic dimer interacting with a barrier; (c) a field of a static electric charge in a conducting cavity surrounded by four spherical segments. Concrete experimental suggestions include (a) an “entanglement amplifierâ€, (a’) integrability induced peaks in a relaxation time vs. mass ratio curve for a binary mass mixture, (b) a novel observable selection rule for some one-dimensional chemical processes and the usage thereof for miniaturization of chip-based atom interferometers, and (c) nineteen three-parametric families of solvable electrostatic problems in piece-wise-spherical cavities with conducting grounded walls. Liens : |
| Ambroise van Reokeghem (CEA Grenoble) | Détails Fermer |
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Transition-metal pnictides : electrons and phonons le mercredi 26 juin 2019 à 11:00 |
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Résumé : The family of iron-based superconductors display signatures of moderate electronic correlations due to strong Hund's coupling, leading to quasiparticle renormalizations and relatively low coherence temperatures. In this seminar, I will discuss the electronic structure of a series of 122 transition metal pnictides, from chromium to copper, based on photoelectron spectroscopy experiments and ab initio calculations. In a second part, I will also discuss the interplay of electronic and lattice effects in a few iron-based compounds. Liens : |
| Zhihui Peng (Hunan Normal University, Changsha, China ) | Détails Fermer |
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Coupling of a Cavity and a Transmission Line with a Superconducting Artiï¬cial Atom le mardi 25 juin 2019 à 14:00 |
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Résumé : We report our experimental results about strong coupling of a cavity and a transmission line with an superconducting artificial atom. With the special architecture, we observed the vacuum-induced Aulter-Townes splitting[1] which has potential application in microwave quantum network. We also observed anomalous resonance fluorescence of an atom-cavity coupled system[2] which is qualitatively different from the driven-atom in free space. Our results show the superconducting artificial atom is an ideal testbed for quantum optics. References: 1. Z.H. Peng et al., PRA 97, 063809 (2018). 2. Z.H. Peng et al., In preparation. Liens : |
| Tommaso Roscilde (ENS-Lyon) | Détails Fermer |
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Assessing many-body quantumness via correlation functions le vendredi 21 juin 2019 à 11:00 |
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Résumé : Decades of research into the foundational aspects of quantum mechanics — started with the Einstein-Podolsky-Rosen (EPR) paradox on entanglement and non-locality — have brought us a radically new way of thinking about physical systems: the latter can be now viewed as hosts of quantum information encoded in quantum superpositions, and as potential resources for novel quantum technologies. A central question is how to assess the non-classical nature of quantum states, namely their characterization as coherent superpositions featuring non-local quantum correlations. This question becomes particularly intriguing and intricate when moving to many-body systems: the exponential growth of quantum information with the system size makes many-body tomography simply inaccessible, and strategies for a scalable assessment of quantumness need to be devised. This endeavor has obviously a foundational aspect, ultimately aiming at an exploration of the mysterious quantum-classical or micro-macro boundary; but it has also immediate bonuses, since assessing quantumness of many-body states can translate into probing their potential use as resources for quantum information tasks. In this talk I will address the question: assuming the one has theoretical or even experimental access to correlation functions related to a generic quantum many-body state (pure or mixed, at equilibrium or far from it), can one make conclusive statements about the quantum nature of the state in question? By "quantum nature†I mean here the various forms of increasing non-classicality, namely entanglement; EPR correlations; and Bell correlations. I will show that bipartite entanglement and EPR correlations can be effectively assessed via the knowledge of correlations between two subsystems; and that they are in fact generic features of systems with continuous symmetries in the “canonical†ensemble (namely at fixed magnetisation for quantum spin systems; fixed particle number for lattice quantum gases). Moreover I will illustrate how one can make an exhaustive Bell test on the measured correlations -- unveiling constructively their definite incompatibility with classical physics — without making use of Bell’s inequalities. Liens : |
| Giovanni (LPMMC) | Détails Fermer |
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Présentations des stages de M2 le mercredi 19 juin 2019 à 11:00 |
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Résumé : Giovanni, Tomaso et Anatole nous présenterons leur stage au LPMMC. (15 minutes de présentation et 5 minutes de question chacun). Liens : |
| Yiftach Frenkel (Bar Ilan University, Israel) | Détails Fermer |
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Scanning SQUID measurements of domain walls in SrTiO3 le mardi 18 juin 2019 à 14:00 |
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Résumé : The interface between the oxide insulators Lanthanum Aluminate and Strontium titanate hosts a gate tunable 2D electron gas that also becomes SC at low temperatures. It has been demonstrated that the 2DEG can be confined to create devices such as gate defined SQUIDs or a single electron transistor. In effect the Physics of the SrTiO3 substrate play a major role in the behavior of the interface. SrTiO3 undergoes a structural phase transition at 105K resulting in a dense network of domains separated by nanometer thick twin walls. I will discuss our recent findings, where we used scanning SQUID microscopy to map the spatial distribution of conduction at the interface. Images of the interface showed quasi-1D channels of modulated current flow, superconductivity and magnetic signal. The domain walls change their location with thermal cycles and with the application of back gate voltage. These findings open exciting possibilities for normal and superconducting devices based on domain walls. Liens : |
| Brijesh Kumar (Jawaharlal Nehru University New Delhi) | Détails Fermer |
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Inversion and Quantum Oscillations in Kondo insulators le vendredi 14 juin 2019 à 11:00 |
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Résumé : Conventionally, the quantum oscillations of magnetisation [the de Haas-van Alphen (dHvA) effect] have come to be exclusively associated with metals. But recent observations of magnetic quantum oscillations in Kondo insulators (SmB6 and YbB12) challenge this conventional view, and call for a reexamination. We study this problem by investigating the basic models of Kondo insulators for their orbital response to uniform magnetic field. By doing a self-consistent theory of the charge dynamics of Kondo insulators in a novel representation for electrons [1], we discover the gapped charge quasiparticles to undergo inversion upon decreasing the Kondo coupling, and establish the inversion to be the key determinant for quantum oscillations to occur as a bulk phenomenon in Kondo insulators [2,3]. The frequency of dHvA oscillations we obtain corresponds to the half of the bulk Brillouin zone, as observed experimentally [4]. References: [1] Brijesh Kumar, Phys. Rev. B 77, 205115 (2008) [2] Panch Ram and Brijesh Kumar, Phys. Rev. B 96, 075115 (2017) [3] Panch Ram and Brijesh Kumar, arXiv:1809.04654; Phys. Rev. B (2019). In production. [4] B. S. Tan et al, Science 349, 287 (2015) Liens : |
| Anjan K. Gupta (Indian Institute of Technology Kanpur) | Détails Fermer |
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Optimization of constriction based niobium µ-SQUIDs for probing nano-magnetism le mardi 11 juin 2019 à 14:00 |
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Résumé : Magnetometry using micron-size superconducting quantum interference devices (µ-SQUIDs) has been remarkably successful in probing classical as well as quantum regimes of magnetism in single nano-particles. This technique can be further improved for higher speed and sensitivity with hysteresis-free µ-SQUIDs. This is difficult, particularly, at low temperatures, which is essential for probing quantum-magnetism. The hysteresis in these devices arises from thermal instabilities in superconducting weak-links and neighboring region. The heat generated in resistive normal region gives rise to a self sustained hot-spot. This leads to two possible states, hot (normal) and cold (superconducting), and hence bistability. Such hot-spot and hysteresis has been modeled in the past by using steady state thermal heat balance equations. However, as we approach the hysteresis-free regime by optimizing the relative heat evacuation, another regime of hysteresis is found in which the bistability results due to a phase dynamic steady state. We understand this dynamic regime using a thermal model that helps us quantitatively capture the behavior in both hysteretic and non-hysteretic regimes. Slow relaxation of quasi-particles, which are generated due to phase dynamics, is found to be a bottleneck, which is the case for several superconducting devices including SIN-coolers and superconducting qubits. We solve the thermal model for different shunting conditions to find that an optimal shunt having resistance and inductance both can eliminate hysteresis at low temperatures and with a good sensitivity. A pure resistive shunt, which works well for hysteresis elimination in usual tunnel junction based SQUIDs, leads to a marked reduction in sensitivity of µ-SQUIDs. This new model also reveals an interesting non-linear dynamical system with various regimes. We successfully test this idea of inductive shunt eliminating hysteresis with good sensitivity. Finally, we present preliminary results on magnetization reversal in permalloy nano-needles by using these optimized non-hysteretic µ-SQUIDs. Liens : |
| Peter Makk (University of Basel & Budapest University of Technology and Economics,) | Détails Fermer |
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Engineering exotic states in graphene heterostructures le mardi 04 juin 2019 à 14:00 |
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Résumé : Graphene is an ideal platform to realize novel, topological states of matter by combining it with other 2D materials using van der Waals stacking. These states include the topological insulator state, the valley Hall state and the appearance Majorana excitations have also been predicted by combining special quantum Hall states with superconducting correlations. Here we show our work towards this direction. First I will show supercurrent measurements in graphene/hBN heterostructures with a Moiré superlattice formed by alinging graphene lattice to the hBN lattice. Using the supercurrent measurement we extract the DOS of the superlattice and investigate the appearance of edge states using interferometry measurements. We also show tunneling spectroscopy measurements in graphene, where we extract the non-equilibrium distribution function and investigate the electron cooling mechanisms in graphene. Finally we comment on the appearance of SOI from TMDC substrates.
- D. Indolese, et al., Phys. Rev. Lett., 121, 137701 (2018) Liens : |
| Martina Hentschel (TU Ilmenau, Allemagne) | Détails Fermer |
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From billiards for light to mesoscopic optics le mardi 28 mai 2019 à 14:00 |
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Résumé : The investigation of the propagation of light in mesoscopic, i.e. often micrometer-scale, systems is a rich subject providing insights ranging from quantum chaos in open systems to new schemes for realizing microlasers. The concept of quantum-classical, here wave-ray, correspondence, proves to be as useful as for electronic mesoscopic systems such as quantum dots. Whereas there the electrons are confined by means of gate voltages, the confinement of light in optical microresonators is due to total internal reflection, leading to billiards for light. There are, however, semiclassical deviations from the naive ray-picture expectation in the reflection and refraction of light at dielectric interfaces yielding for example to deviations from Snell's law. We illustrate these effects and discuss their impact on the far-field emission characteristics of optical microcavities. The propagation of electromagnetic waves in three-dimensional optical microcavities requires to pay attention to the evolution of the light's polarization as a new degree of freedom. In systems like dielectric Möbius-strips or cone-shaped microtube cavities, the polarization state of resonant whispering gallery-type modes may differ strongly from the reference case of homogeneous cylinders. Whereas we find that the polarization of the electromagnetic field follows the wall orientation in thin Möbius strips, thereby reflecting the accumulated geometric phase, we observe that the electromagnetic field ignores the Möbius topology when the strip thickness is increased. Breaking of symmetries further influences the morphology of resonances and can induce a transition from linear to elliptical polarization that is both of theoretical interest from the point of view of spin-orbit interaction of light and their interpretation in terms of Berry phases, and relevant for potential applications. Liens :Martina Hentschel |
| CPTGA 24 mai (Café (IPN Orsay) | Détails Fermer |
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Superfluidity in the inner crust of neutron stars le vendredi 24 mai 2019 à 11:00 |
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Résumé : After a general introduction to neutron stars, I will focus on the special role of their inner crust. This region is characterized by the coexistence of a Coulomb lattice of neutron-rich nuclei ("clusters") in a uniform background of ultrarelativistic degenerate electrons and a gas of unbound neutrons. The unbound neutrons are supposed to be superfluid, which gives rise to remarkable phenomena, such as the famous "glitches" (sudden increases of the neutron star's rotation frequency) and changes in the star's cooling behaviour. However, making reliable predictions for the superfluid critical temperature remains a challenging problem for nuclear many-body theory, mostly because of medium-polarisation effects. Another important unsolved problem is the "entrainment" between the neutron gas and the clusters in the crust, since it determines the density of superfluid neutrons. Liens : |
| Christopher Bauerle (Institut Néel) | Détails Fermer |
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In-flight manipulation of single electrons le mardi 21 mai 2019 à 14:00 |
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Résumé : Over the past decade, an important effort has been made in the field of low-dimensional electronic conductors towards single-electron electronics with the goal of gaining coherent control over single flying electrons in solid-state devices [1]. In this talk I will present our recent advances towards the realization of electronic flying-qubit architectures using ultrashort charge pulses (Levitons) as well as surface acoustic wave (SAW) driven single electrons. In the first part of the talk I will present time-resolved measurements of ultrashort single-electron charge pulses injected into a quasi-one-dimensional quantum conductor. We show that the velocity of such a single-electron pulse is found to be much faster than the Fermi velocity due to the presence of strong electron-electron interactions and can be tuned over more than an order of magnitude by electrostatic confinement. In addition, our set-up allows us to tune our system continuously from a clean one-channel Tomonaga-Luttinger liquid to a multi-channel Fermi liquid [2]. Our results are in quantitative agreement with a parameter-free theory and demonstrate a powerful new probe for directly investigating real-time dynamics of fractionalisation phenomena in low-dimensional conductors. In the second part of the talk, I will concentrate on SAW-assisted single-electron transport. I will present our recent results on highly-efficient electron routing in a beam-splitter configuration. For this we connect four quantum dots via two 22 μm long quantum rails that are coupled by a tunnel barrier along a 2 µm long interaction region. Changing the energy detuning in the interaction region we can partition the electrons on-demand into two paths with electron transport efficiencies of 99.7 % [3]. Our results demonstrate the potential of these two approaches for the implementation of an electronic solid-state flying qubit having high relevance in fundamental research and quantum information technology.
[1] C. Bäuerle et al., Rep. Prog. Phys. 81, 056503 (2018) Liens : |
| Raphaël Chétrite (Laboratoire Dieudonné, Université de Nice - Sophia Antipolis) | Détails Fermer |
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Analytical Large Deviation and Uncertainly Relation le vendredi 17 mai 2019 à 11:00 |
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Résumé : In this talk, I will talk about the theory of large deviations. After a general introduction, I will present some recent developments on the large deviations associated with a Markov process and on applications for thermodynamic uncertainty relations. Liens : |
| Marcin Napiorkowski | Détails Fermer |
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Bogoliubov Theory at Positive Temperatures le vendredi 10 mai 2019 à 11:00 |
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Résumé : I shall discuss the homogeneous Bose gas at positive temperatures within Bogoliubov theory. The theory arises by restricting the Hilbert space to quasi-free states. I will introduce the free energy functional and discuss the existence of equilibrium states, phase diagram and critical temperature. This is joint work with Robin Reuvers and Jan Philip Solovej. Liens : |
| Gwendal Feve (ENS) | Détails Fermer |
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Probing quantum Hall conductors with low and high frequency noise le mardi 30 avril 2019 à 14:00 |
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Résumé : In my seminar, I will discuss measurements of low and high frequency noise in quantum Hall conductors and how they can probe the elementary excitations propagating along the edge channels in the integer and fractional cases. I will first present how low frequency noise measurements can be used to extract the electronic states propagating along the edge channels of the integer quantum Hall regime. Combining two-electron Hong-Ou-Mandel interferometry [1] with signal processing techniques, we have implemented a quantum tomography protocol [2,3] able of extracting from any electrical current the generated electron and hole wavefunctions as well as their emission probabilities In the second part of my presentation, I will discuss the measurement of high frequency noise [4] generated by the random transfer of fractional excitations through a potential barrier biased with a dc voltage Vdc. At high frequencies (few GHz), the emitted noise can be interpreted as the generation of microwave photons in a coaxial measurement line weakly coupled to the sample. We observe that photons are only emitted when their frequency is smaller than the frequency threshold fJ=qVdc/h called the Josephson frequency [5,6] in analogy with the Josephson relation in superconductors. This threshold provides a direct determination of the fractional charge q.
[1] E. Bocquillon et al. Science 339, 1054 (2013) Liens : |
| Ambroise van Roekeghem (CEA Grenoble) Annulé | Détails Fermer |
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Transition-metal pnictides: electrons and phonons le mardi 30 avril 2019 à 11:00 |
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Liens : |
| Gabriel Polinario (Université fédérale de Rio, Brésil) | Détails Fermer |
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Onset of intermittency in stochastic Burgers hydrodynamics le mardi 16 avril 2019 à 11:00 |
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Résumé : A number of theoretical efforts have been devoted to the study of intermittent fluctuations of fluid dynamic observables in the stochastic Burgers model, where the presence of velocity shocks leads to large negative fluctuations of the velocity gradient. I am going to discuss how the response functional approach, where specific velocity field configurations - the instantons - are conjectured to be the dominant strucutures for a statistical account of large negative fluctuations, is meaningful only if the effects of fluctuations around instantons are taken into account. |
| CPTGA 12 avril (Café (Institut Langevin) | Détails Fermer |
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Acoustic bubbly metamaterials: subwalength focusing, negative refraction and super-absorption le vendredi 12 avril 2019 à 11:00 |
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Résumé : I will show that air bubbles, in water or trapped in a soft solid, are excellent candidates for creating acoustic metamaterials. They indeed exhibit a strong low-frequency monopolar resonance, which can lead to interesting effective acoustic properties at wavelengths that can be hundreds of times larger than the radius of the bubbles. First, I will show the possibility of focusing inside a bubbly metamaterial with a subwavelength resolution. The demonstration will be based on numerical results obtained with a Multiple Scattering Theory (MST) code that fully incorporates multiple-scattering effects. Then, I will explain how to create a 3D disordered double negative metamaterial composed solely of monopolar resonators. Finally, I will demonstrate that acoustic superabsorption can be achieved over a broad frequency range by tuning the parameters of a single layer of bubbles, referred to as a metascreen, which is confirmed by both finite element simulations and experiments. Liens : |
| Michael Pasek | Détails Fermer |
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Density-wave steady-state phase of dissipative ultracold fermions with nearest-neighbor interactions le mercredi 10 avril 2019 à 13:30 |
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Résumé : I will describe our recent results about the effect of local dissipation on density-wave ordering in the extended Fermi-Hubbard model with both local and nearest-neighbor interactions. For this purpose, we used a recent variant of nonequilibrium dynamical mean-field theory with the auxiliary master equation approach which allows to treat nonperturbatively both local dissipation and local interaction. I will show how density-wave order seems to be robust against dephasing effects up to a critical point, where the system becomes homogeneous with no spatial ordering. I will also show how this model can be realized in ultracold atom experiments by the dressing of fermionic atoms with highly-excited Rydberg states in an optical lattice. Liens : |
| Alexandra Sheremet (ESPCI) | Détails Fermer |
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Coherent control of light transport in a dense atomic medium le mercredi 10 avril 2019 à 11:00 |
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Résumé : Light-matter interfaces play a crucial role in the context of quantum information networks, enabling for instance the reversible mapping of quantum state of light onto quantum states of matter. A promising approach for the realization of such interfaces is based on ensemble of neutral atoms. A critical figure of merit of such interfaces is the overall storage-and-retrieval efficiency, which is mainly determined by technical losses and atomic decoherence, and depends on the storage mechanism and matter properties. Collective and cooperative effects manifistable in an atomic ensemble could provide essential enhancement of the coupling strength between the light and atomic systems. In this context, one of the strongest requirements to obtain a high efficiency is a large optical depth, which can be achieved by increasing the size of the atomic system or atomic density in the system. In addition, the interaction between light and atoms can be enhanced by trapping atoms in the vicinity of a nanoscale waveguide due to strong confinement of the light. In this talk I will discuss light propagation in a spatially dense atomic ensemble, where the average distance between atoms is comparable with the resonant wavelength. In such dense atomic configurations dipole-dipole interaction play an important role and can lead to manifestation of super and subradiance effects. I will consider the light propagation in both free space and trapped near nanofiber surface atomic ensembles. The light scattering in such dense atomic configuration is described in terms of microscopic approach based on the standard scattering matrix and Resolvent operator formalism. We show theoretically and experimentally that spatially dense atomic ensembles allow obtaining effective light-matter interface and reliable light storage with essentially fewer atoms than it can be achieved in dilute gases. Furthermore, we show that the presence of an optical nanofiber modifies the character of atomic interaction and results in long-range dipole-dipole coupling between atoms not only via the free space, but also through the waveguide mode. Liens : |
| MISSING | Détails Fermer |
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(titre non communiqué) le mardi 09 avril 2019 à 14:00 |
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Liens : |
| Julien Toulouse | Détails Fermer |
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Rigorous combination of wave-function methods and density-functional theory for electronic-structure calculations le vendredi 05 avril 2019 à 11:00 |
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Résumé : I will first give a brief overview of the main goals of quantum chemistry and of the two families of electronic-structure computational methods used to solve the many-electron Schrödinger equation in this domain, namely wave-function methods and density-functional theory. I will then explain the advantages of combining these two approaches and how this can be done in a rigorous way based on a partition of the Coulomb electron-electron interaction into long-range and short-range contributions. The idea is to use a many-body wave-function method for the long-range contribution, coupled with a density-functional approximation for the short-range contribution. I will show two specific realizations of this range-separated wave-function/density-functional theory using for the wave-function method: 1) a random-phase approximation, which allows us to describe van der Waals intermolecular interactions; 2) a selected configuration-interaction approach, which allows us to describe strong electron correlation effects. Liens : |
| Matthieu Tissier (LPTMC) | Détails Fermer |
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Critical properties of the Random field Ising Model le mercredi 03 avril 2019 à 11:00 |
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Résumé : The random field Ising model is a classic of statistical mechanics, which was proposed more than 40 years ago by Imry and Ma. Because of its simplicity, it is relevant for describing many physical situations, both at equilibrium and out-of-equilibrium. After describing some of these experimental realizations, I will present the most striking features that were encountered in the theoretical study of this model (dimensional reduction and its breaking, static avalanches ...). I will explain what are the minimal ingredients needed to describe such situations from an analytic perspective. I will finally present the results we obtained in the last decade, by making use of the functional renormalization group. Liens : |
| Daniel Szombati (University of Queensland) | Détails Fermer |
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Quantum rifling and some quantum goodies from hybrid structures le mardi 26 mars 2019 à 13:30 |
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Résumé : Quantum mechanics postulates that a measurement forces the wave-function of a qubit to collapse to one of its two eigenstates. The result of the measurement can then be recorded as a discrete outcome designating the particular eigenstate the qubit collapsed to. I will show that this well-accustomed picture of quantum measurement breaks down when the qubit is strongly driven during measurement. More specifically, when the evolution speed exceeds a threshold defined by the characteristic measurement time, the measurement outcome does not contain any information about the initial state of the qubit and thus the measurement does not generate any back-action. We call this phenomenon quantum rifling, as the fast spinning of the Bloch vector protects it from being deflected into either of its two eigenstates. We study this phenomenon with two superconducting qubits coupled to one to the same detector and demonstrate that the quantum rifling allows us to measure either one of the qubits on demand while protecting the state of another one from the measurement back-action.
If time permits it, I will also cover the results of my PhD from TU Delft, where I have been studying the Josephson effect in semiconducting InSb nanowires. These nanowires bare exotic electronic properties, such as large g-factor and spin-orbit interaction, leading to peculiar behaviour of the Josephson supercurrent. Specifically, the switching current exhibits non-monotonic behaviour with increasing magnetic field, due to the orbital interference of many modes in the wire[1]. For certain magnetic field values, we observe supercurrent flowing at zero phase difference, otherwise known as a Josephson-phi0 junction[2]. Such phi0-junctions can serve as smoking gun signatures of Majorana fermions.
Liens : |
| Xavier Montiel (University of London) | Détails Fermer |
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Generation of pure superconducting spin current in superconducting heterostructures via non-locally induced magnetism le mercredi 20 mars 2019 à 13:30 |
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Résumé : Superconducting spintronics aims at carrying spin currents via equal spin Cooper pairs in superconducting/ferromagnetic heterostructures [1]. In this talk, I will present a mechanism for the generation of pure superconducting spin-currents carried by equal-spin triplet Cooper pairs in a superconductor (S) sandwiched between a ferromagnet (F) and a normal metal (Nso) with intrinsic spin-orbit coupling [2]. I will show that in the presence of Fermi-liquid interactions, the superconducting proximity effect can induce non-locally a ferromagnetic exchange field in the normal layer, which disappears above the superconducting transition temperature of the structure. The internal Fermi-liquid exchange field leads to the onset of a spin supercurrent associated with the generation of long-range spin-triplet superconducting correlations in the trilayer. I will show that the magnitude of the spin supercurrent, as well as the induced magnetic order in the Nso layer, depends critically on the superconducting proximity effect between the S layer and the F and Nso layers and the magnitude of the relevant Landau Fermi-liquid interaction parameter. These results provide a mechanism for the generation of equal spin Cooper pairs that is compatible with recent experimental results [3]. I will also give a brief discussion on our ongoing work on non-equilibrium spin currents in superconducting structures.
Liens : |
| Benjamin Lenz | Détails Fermer |
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Effects of non-local correlations on spectral properties of doped Sr2IrO4 le mercredi 20 mars 2019 à 11:00 |
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Résumé : The spin-orbit Mott insulator Sr2IrO4 has been in the spotlight in recent years due to its striking similarities to isostructural high-Tc superconducting copper oxides. In particular, upon doping the system recent photoemission experiments found pseudogap behavior at low temperatures, which raises the question of its relation to the pseudogap found in cuprate superconductors. In this talk, I will present new insights into the spectral properties of this 5d transition metal system as a function of electron- and hole-doping by means of a combined ab-initio electronic structure and oriented cluster dynamical mean-field approach. Within this treatment, important ingredients like spin-orbit coupling and distortions of the oxygen octahedra as well as Hubbard interactions and non-local charge fluctuations are taken into account. The calculated spectral function of pure Sr2IrO4 compares well with angular-resolved photoemission measurements, both in the low-temperature antiferromagnetic and high-temperature paramagnetic phase, and allows to study emerging changes under electron- and hole-doping. Special emphasis of my talk will be placed on pseudogap features of the momentum-resolved spectral function of electron-doped Sr2IrO4, which are found to be in good agreement with experiment. |
| Oleksandr Tsyplyatyev (Francfort University) | Détails Fermer |
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A hierarchy of strongly correlated modes in quantum wires le vendredi 15 mars 2019 à 11:00 |
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Résumé : The natural excitations of an interacting one-dimensional system at low energy are hydrodynamic modes of Luttinger liquid, protected by the Lorentz invariance which originates from the linearised dispersion. In this talk, I will show that beyond low energy, where quadratic dispersion reduces the symmetry to Galilean, the organisational principle of the many-body excitations changes into a hierarchical structure: calculations of dynamic correlation functions for fermions show that the spectral weights of the excitations are proportional to integral powers of R^2/L^2, where R is the interaction radius and L is the system length. Thus, only small numbers of excitations carry the principal spectral power in representative regions on the energy-momentum planes. For example, in the spectral function the first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalised single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power-laws at the spectral edge. Crossover from this hierarchy in the nonlinear regime to Luttinger liquid at low energy will be illustrated by a calculation of the local density of state at all energy scales using Bethe ansatz. I will also give a brief discussion of experiments on quantum wires realised in GaAs double-well heterostructures. The momentum-resolved tunnelling in this setup directly probes the spectral function of electrons at all energy scales giving access to the spin-charge separation of spinful Luttinger liquid in the linear and to the hierarchy of strongly correlated modes in the nonlinear regime. Liens : |
| Hadrien Kurkjian (Anvers) | Détails Fermer |
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Modes collectifs de "Higgs" dans les condensats fermioniques le mercredi 13 mars 2019 à 11:00 |
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Liens : |
| Hélène Sueur (CSNSM, Univ. Paris-Sud) | Détails Fermer |
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Microscopic charged fluctuators as a limit to the coherence of disordered superconductor devices le mardi 12 mars 2019 à 14:00 |
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Résumé : By performing experiments with thin-film resonators of NbSi, we elucidate a decoherence mechanism at work in disordered superconductors. This decoherence is caused by charged Two Level Systems (TLS) which couple to the conduction electrons in the BCS ground state; it does not involve any out-of-equilibrium quasiparticles, vortices, etc. Standard theories of mesoscopic disordered conductors enable making predictions regarding this mechanism, notably that decoherence should increase as the superconductor cross section decreases. Given the omnipresence of charged TLS in solid-state systems, this decoherence mechanism affects, to some degree, all experiments involving disordered superconductors. In particular, we show it easily explains the poor coherence observed in quantum phase slip experiments and may contribute to lowering the quality factors in some disordered superconductor resonators. Liens : |
| David Hagenmüller (ISIS) | Détails Fermer |
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Shaping the properties of condensed-matter systems with light le mercredi 06 mars 2019 à 11:00 |
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Liens : |
| Boris Brun (UCLouvain, Belgium) | Détails Fermer |
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Imaging thermoelectric transport through quantum nanostructures le mardi 05 mars 2019 à 14:00 |
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Résumé : We developed a new scanning probe technique to image thermoelectric transport in two- dimensional devices: Thermoelectric Scanning Gate Microscopy (TSGM). This technique is derived from Scanning Gate Microscopy (SGM), that consists in mapping changes in a device's electrical conductance induced by a moving electrostatic perturbation, generated with a biased AFM tip [1]. TSGM consists in recording the devices’ Seebeck coefficient instead of its electrical conductance. To perform this measurement, we heat one side of the device and record the thermoelectric voltage arising across the device in response to this temperature difference. We then scan the electrically biased tip above the surface while recording this signal. We apply this technique to investigate the low density regime of quantum point contacts (QPCs), where strong electron-electron interactions give rise to conductance [2,3] and thermoeletcric [4] anomalies. By scanning the polarized tip in front of the QPC, we create a Fabry-PeÌrot cavity between the QPC channel and the tip-depleted region [5], which induces interference fringes in both the conductance and the thermopower. Surprisingly, the interference in the thermoelectric signal exhibit an abrupt phase shift by Ï€ at very low QPC transmission, which is invisible in the conductance. We propose a model to explain these differences, based on the spontaneous localization of electrons in the QPC channel [6,7]. Our work illustrates that the combination of scanning gate microscopy and thermoelectric measurements can unveil elusive phenomena that escape transport measurements [8]. [1] M.A Topinka et al., Nature, 416, 183-186 (2001). [2] K.J Thomas, Phys. Rev. Lett. 77, 135 (1996). [3] S. M. Cronenwett, Phys. Rev. Lett. 88, 226805 (2002). [4] N. J. Appleyard, Phys. Rev. B 62, 8 R16275 (2000) [5] B. Brun et al., Phys. Rev. Lett. 116, 136801 (2016). [6] M. J. Iqbal et al. Nature, 501, 79 (2013) [7] B. Brun et al., Nat. Com., 5, 4290 (2014) [8] B. Brun et al., arXiv:1804,00075 Liens :Boris Brun |
| CPTGA 1er mars (Café (IAP and ILP, Sorbonne University, Paris, CCA, Flatiron Institute, New York) | Détails Fermer |
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Confronting Theory and Data in Cosmology le vendredi 1er mars 2019 à 11:00 |
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Résumé : The mysteries of the cosmic beginning, gravitational clustering, and cosmic acceleration persist. How can we distill relevant cosmological information from the next generation of data sets? Taking examples from the cosmic microwave background, large scale structure, and supernova cosmology, I will discuss inference strategies, artificial intelligence, machine learning, and computational approaches that promise to extract more information from current and upcoming data sets. The philosophy is to allow maximum freedom to design realistic forward models, to be robust to systematic nuisances, accurately combine multiple probes, move beyond simplistic likelihood assumptions, naturally allow quantitative model comparison, characterize tensions in the data, and maintain (near-)optimality whenever possible. Liens : |
| Serena Cenatiempo (Gran Sasso Science Institute) | Détails Fermer |
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Bogoliubov theory in the Gross-Pitaevskii regime le vendredi 15 février 2019 à 11:00 |
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Résumé : Since 1947 Bogoliubov theory has represented the guide model to thinking about weakly interacting Bose gases. Remarkably, such a theory predicts a linear excitation spectrum and provides expressions for the thermodynamic functions which are believed to hold in the dilute limit. However, so far, there are only a few cases where the predictions of Bogoliubov theory can be obtained by rigorous mathematical analysis. In particular, one of the main mathematical issues is to recover the physical intuition that the correct parameter to appear in the expressions of the physical quantities is the scattering length of the interaction. In this talk I will discuss how the validity of Bogolibov theory can be proved in the case of systems of N interacting bosons trapped in a box with volume one and interacting through a repulsive potential with scattering length 1/N (Gross-Pitaevskii regime). This is a joint work with C. Boccato, C. Brennecke and B. Schlein. Liens :Serena Cenatiempo |
| Simon Pigeon | Détails Fermer |
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Turbulent flow and soliton interaction in resonantly-driven polaritons superfluids le mercredi 13 février 2019 à 13:30 |
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Résumé : Exciton-polaritons, microcavity half-matter half-light quasi-particles, when resonantly driven exhibit a superfluid regime. Accordingly, topological excitations similar to those predicted in equilibrium superfluids may spontaneously appear [1,2]. However, the non-equilibrium nature of polaritons requires the system to be continuously pumped to compensate for losses. This driving plays a crucial role in the formation and dynamics of such topological excitations tending to inhibit their formation [1]. I will present a recent breakthrough allowing to simultaneously extended the fluid propagation distance and to release the constraints imposed by the resonant driving [3]. This fully optical method, exploiting optical bistability present in these systems, allows for accurate hydrodynamics study of polariton superfluid and for a deterministic control of excitation taking place is this unconventional fluid of light. Experimental validation of the proposal will be reported. I will also discuss prospects open thanks to this method towards non-linear statistical physics and quantum correlation. Liens : |
| Irénée Frérot | Détails Fermer |
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Quantum correlations close to quantum critical points : entanglement and beyond le mercredi 13 février 2019 à 11:00 |
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Résumé : Second-order quantum phase transitions host coherent superpositions and entanglement at all length scales. Theoretically, it is expected that a quantum critical point (QCP) induces a specific scaling of thermodynamic observables in an extended region of the finite-temperature phase diagram (the so-called quantum critical fan). So far, characterizing the extent of the quantum critical fan has however proved challenging, given the interplay of quantum and thermal fluctuations around the QCP. After introducing a simple procedure to isolate the coherent contribution to the fluctuations of an arbitrary observable, we will propose such a characterization for paradigmatic spin models of quantum phase transitions. In a second part of the talk, we will explain why the entanglement generated close to a QCP is a potential resource for quantum interferometry, and illustrate this general property by describing a counter-intuitive, genuinely quantum, mechanism, for the suppression of certain fluctuations at the QCP of the quantum Ising model (leading to spin-squeezing). Liens :Irénée Frérot |
| MISSING (Jussieu) Annulé | Détails Fermer |
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Radiative corrections in planar Dirac liquids le vendredi 08 février 2019 à 11:15 |
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Annulé
Liens : |
| Antonin Coutant | Détails Fermer |
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Black holes in fluid flows le mercredi 06 février 2019 à 13:30 |
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Résumé : I will discuss the possibility to reproduce black hole physics in fluid flows. The starting point is an analogy discovered by Unruh between the propa- gation of sound in a flowing fluid and waves around a black hole. I will discuss the analogue of the Hawking effect through which a black hole loses its mass, and its recent experimental verifications. I will also present a recent water wave experiment, where we have observed the analogue of black hole superradiance, that is, the amplification of waves by extraction of angular momentum from a rotating flow. Liens : |
| MISSING (Université de Cergy-Pontoise) | Détails Fermer |
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Quantum many-body physics with nonlinear propagating light le mercredi 06 février 2019 à 11:00 |
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Liens : |
| CPTGA 1er février (Café (Peter Grünberg Institute) | Détails Fermer |
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Precision couplings and tailored couplings for high-fidelity quantum computing le vendredi 1er février 2019 à 11:00 |
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Résumé : This is a two-part talk, one is about consideration of longitudinal rather than transverse coupling for qubit-resonator systems, and the other part is about a critical examination of the rotating wave approximation. Liens : |
| Rémy Dubertrand (Ratisbonne) | Détails Fermer |
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A semiclassical perspective for quantum many-body systems le mercredi 30 janvier 2019 à 13:30 |
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Résumé : One way to characterize complex quantum systems is to consider their corresponding classical counterpart. Such a program, dubbed as quantum chaos, has been highly successful for low-dimensional (mainly one-body) systems. There is a growing interest in transferring these techniques towards many-body systems. First I will explain how to describe the energy spectrum and the associated eigenstates for the seminal Bose-Hubbard system. Second I will consider the disordered Anderson problem on a random graph. I used extensive numerical techniques in order to characterise the localised/delocalised transition and whether the delocalised phase is ergodic. Liens : |
| Nicolas Macé (IRSAM Toulouse) | Détails Fermer |
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Many-body localization and thermalization in one-dimensional quantum systems le mercredi 30 janvier 2019 à 11:00 |
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Résumé : At high energy, isolated quantum systems generically thermalize, their macroscopic observables obeying the classical laws of thermodynamics. In many-body localized systems however, transport is prevented, effectively breaking thermalization. The phenomenon of many-body localization (MBL) is interesting (1) from a applied point of view, to engineer states robust to decoherence effects, and (2) from a fundamental point of view, as a genuinely quantum phenomenon whose understanding is instrumental in crafting a quantum theory of thermodynamics. In this talk, I will review the progresses made in that direction, notably discussing the minimal ingredients needed for MBL to arise, and presenting a picture of the MBL phase as a fractal delocalized phase on a complex graph. If time permits, I will extend the discussion to out-of-equilibrium open systems.
Selected references
Liens : |
| Dmitry Bagrets (University of Cologne) | Détails Fermer |
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The Sachdev-Ye-Kitaev model, its holographic dual and quantum conformal fluctuations le vendredi 25 janvier 2019 à 11:00 |
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Résumé : The fascinating Sachdev-Ye-Kitaev (SYK) model describing a large number of randomly interacting Majorana fermions represents an ultimate example of the AdS/CFT correspondence in 1 et 1 space-time dimensions. As pointed out by Kitaev, both the SYK model and its gravity dual possess an emergent conformal symmetry which is spontaneously broken in the infra-red. As such, the soft Goldstone mode in the spectrum of the model emerges which is known to be described by the so-called 'Schwarzian' action. In my talk, after a general exposition to the SYK model, I will concentrate on its quantum deeply infra-red limit where conformal Goldstone fluctuations start to play a paramount role. I will demonstrate how the 'Schwarzian' action can be mapped onto a 'Liouvillian' quantum mechanics and study a long-time limit of 2- and 4-point correlation functions of Majoranas. The range of new results predicted by such mapping encompasses universal power-law decays of correlators as well as an emergent Coulomb blockade physics in the 'complex' version of the SYK model reminiscent to that of conventional mesoscopic quantum dots. Liens : |
| Elisa Rebolini (ILL) | Détails Fermer |
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Range-separated DFT for molecular excitation energies le mercredi 23 janvier 2019 à 13:30 |
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Liens : |
| Giovanni Martone (LPTMS, Orsay) | Détails Fermer |
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Static and dynamic properties of spin-orbit-coupled Bose gases le mercredi 23 janvier 2019 à 11:00 |
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Résumé : The realization of synthetic spin-orbit coupling represents one of the most important achievements in the physics of ultracold atomic gases. In this talk I shall illustrate some of my theoretical predictions about the properties of two-component Bose-Einstein condensates with equal-weighted Rashba and Dresselhaus spin-orbit couplings. Their phase diagram includes different structures, such as a spin-polarized plane-wave phase, and a stripe phase featuring density modulations. Because of the simultaneous presence of superfluidity and of a crystalline structure, the stripe phase exhibits the long-sought phenomenon of supersolidity. Several relevant features of this configurations, recently observed in an experiment by Ketterle’s group at MIT, will be discussed. Liens :LPTMS, Orsay |
| Alastair Abbott (Genève) | Détails Fermer |
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Quantum Information Beyond the Circuit Model le mercredi 16 janvier 2019 à 11:00 |
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Résumé : The standard circuit model to quantum information has proven a powerful tool not just for quantum computation, but understanding quantum communication tasks, quantum networks and diverse problems including, e.g., quantum metrology. Recently, the potential of going beyond the circuit model, for example by applying operations in superpositions of different orders, has garnered interest, showing new advantages in some computational and communication tasks. I will discuss some of the advantages that can be obtained in such "indefinite causal structures†and the challenges involved in using this as a resource for quantum information. I will finish by briefly mentioning another related approach that may allow some similar advantages while remaining within a causal framework. Liens : |
| Guillaume Lévy | Détails Fermer |
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Graphes quantiques, trou spectral et optimisation des formes le mercredi 19 décembre 2018 à 11:00 |
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Résumé : Dans cet exposé, on considère le laplacien unidimensionnel agissant sur des fonctions définies sur un graphe métrique. En guise de condition aux sommets, analogue dans ce cadre des conditions au bord, on impose les conditions de Neumann, qui traduisent la conservation du courant à travers les sommets. Pour un graphe de départ donné, on s'autorise à faire varier les longueurs des arêtes tout en conservant la longueur totale du graphe. On cherche ensuite des bornes explicites sur la première valeur propre non nulle de cet opérateur (égale au trou spectral ici) en fonction de paramètres métriques, topologiques et combinatoires du graphe sous-jacent, que l'on souhaite optimales, ainsi que les graphes atteignant les valeurs extrêmes. On résout ainsi complètement le problème du minimum et l'on apporte également des réponses partielles au problème du maximum. On conclut en présentant une conjecture sur la forme générale des maximiseurs. Ce travail a été réalisé en collaboration avec Ram Band, du Technion (Haïfa). Liens : |
| Vijay Singh (Université de Hamburg) | Détails Fermer |
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Sound-propagation and superfluidity in ultracold quantum gases le mardi 18 décembre 2018 à 11:00 |
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Résumé : Ultracold atom systems are well-controlled and tunable quantum systems, and thereby enable us
to explore many-body quantum effects, such as BEC-BCS crossover, or superfluidity. In this talk, I
will examine sound-propagation and superfluidity in ultracold quantum gases using analytical and
simulation techniques. I will report on the second sound measurements in the BEC-BCS crossover
and their theoretical analysis [1]. Here, I will demonstrate that the second sound velocity vanishes
at the superfluid-thermal boundary, which is a defining feature of second sound. I will then study
sound propagation in a uniform 2D Bose gas across the superfluid-thermal transition, motivated
by the recent experiments in the Dalibard group. As a key result, I will show that the sound
velocity undergoes a qualitatively different temperature dependence in the critical regime, which
occurs due to an interplay between the superfluid and thermal mode [2]. In the second part of this
talk, I will investigate superfluidity in ultracold quantum gases via laser stirring. I will present the
stirring experiments in the BEC-BCS crossover and provide a quantitative analysis of the onset of
dissipation associated with the breakdown of superfluidity [3]. I will then explore superfluidity across
the Kosterlitz-Thouless (KT) transition with laser stirring and provide a quantitative understanding
of the 2D stirring experiments performed in the Dalibard group [4]. I will also present the noise
correlations of the time-of-flight images of 2D clouds and use them to determine the phase coherence
of the recent experiments at Hamburg [5].
References[1] D. Hoffmann, V. P. Singh, T. Paintner, W. Limmer, L. Mathey, and J. H. Denschlag, “Second sound in the BEC-BCS crossoverâ€, forthcoming. [2] V. P. Singh, and L. Mathey, “Sound propagation in a 2D Bose gas across the superfluid transitionâ€, forthcoming. [3] W. Weimer, K. Morgener, V. P. Singh, J. Siegl, K. Hueck, N. Luick, L. Mathey, and H. Moritz, Phys. Rev. Lett. 114, 095301 (2015); V. P. Singh et al., Phys. Rev. A 93, 023634 (2016). [4] V. P. Singh, C. Weitenberg, J. Dalibard, and L. Mathey, Phys. Rev. A 95, 043631 (2017). [5] V. P. Singh and L. Mathey, Phys. Rev. A 89, 053612 (2014). Liens : |
| CPTGA 14 décembre (Café (Laboratoire Kastler Brossel, Ecole Normale Supérieure, Paris, France.) | Détails Fermer |
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Dual Bose Fermi Superfluids: surprises in the ultracold world le vendredi 14 décembre 2018 à 11:00 |
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Liens : |
| Ilya Tokatly (European Theoretical Spectroscopy Facility (ETSF), San Sebastian, Spain) | Détails Fermer |
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A unified view of spin-charge coupling in classical and superconducting spintronics: Non-disipative Magnetoelectric Effects le vendredi 30 novembre 2018 à 11:00 |
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Résumé : This talk presents an overview of the theory of non-dissipative magnetoelectric effects in superconducting structures with spin-orbit coupling (SOC). I will emphasize a close connection between “classical spin-orbitronics†effects in normal systems and coherent transport phenomena mediated by SOC in superconductors. I start with an introduction to magnetoelectric effects in normal conductors with SOC, and to the standard quasiclassical theory of superconducting structures. By combining ideas from these two fields I will develop a theory of diffusive superconductors in the presence of SOC, and analyze a number of specific magnetoelectric effects. In particular we consider the generation of the long-range triplet condensate, the supercurrent induced spin/triplet accumulation, and the anomalous Josephson effect. We will see that these effects are the direct phase-coherent counterparts of the persistent spin helix, the spin Hall effect, the Edelstein effect, and the spin-galvanic effect, which are known for normal conductors. Liens :Ilya Tokatly |
| Olmo Francesconi (LPMMC) | Détails Fermer |
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The Density of States approach to the sign problem in Lattice QFTs. le mercredi 28 novembre 2018 à 11:00 |
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Liens :LPMMC |
| Jürgen Berges (University of Heidelberg) | Détails Fermer |
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Universality far from equilibrium: From the early universe to ultracold quantum gases le mercredi 21 novembre 2018 à 10:00 |
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Résumé : |
| Nicolas Victorin (LPMMC) | Détails Fermer |
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Nonclassical states in strongly correlated bosonic ring ladders le mercredi 14 novembre 2018 à 11:00 |
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Liens :LPMMC |
| Rama Chitra (ETH Zurich) | Détails Fermer |
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Parametric resonance - sensors to many-body phases of matter le vendredi 09 novembre 2018 à 11:00 |
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Liens : |
| Stefanos Kourtis | Détails Fermer |
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Spectroscopic signatures of topology in the particle-hole continuum of nodal-point semimetals le vendredi 26 octobre 2018 à 11:00 |
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Résumé : In this talk, I will discuss how interesting features of topological origin in material band structures can be inferred by mapping the particle-hole continuum with spectroscopic probes. I will first highlight some simple geometric characteristics of the particle-hole continuum that arise due to nodal points in the electronic dispersion. These characteristics can be used to infer the presence of Weyl and Dirac nodes both below and above the Fermi level in bulk three-dimensional bands, and can thus serve as identifiers of topological content. I will then argue that resonant inelastic x-ray scattering (RIXS) is an appealing tool for directly accessing and studying the features of the particle-hole continuum of topological semimetals, especially in settings that preclude the use of other probes, such as, e.g., in magnetic fields. Going a step further, I will discuss how polarization-resolved RIXS can be used to visualize the Berry curvature distribution around Weyl nodes. Most importantly, I will show how this method can determine the topological charge of individual Weyl nodes, indicating its potential for a spectroscopic measurement of a quantized topological invariant. I will conclude with a quick overview of ongoing experimental and theoretical work. Liens : |
| Jacopo De Nardis | Détails Fermer |
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Generalized Hydrodynamics in integrable chains: non-equilibrium steady states, Drude weights and diffusive spreadings le mercredi 24 octobre 2018 à 11:00 |
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Résumé : We show how the recently developed generalized hydrodynamic theory - an exact hydrodynamic description of the inhomogenous non-equilibrium time evolution of integrable models - has been very efficient to
Liens : |
| Nicola Lo Gullo (QTF Centre of Excellence,Department of Physics and Astronomy, University of Turku Finland) | Détails Fermer |
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Slow dynamics of interacting ultracold gases in aperiodic geometries: a non-equilibrium Green’s function approach le vendredi 19 octobre 2018 à 11:00 |
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Liens : |
| Alessandro 17 octobre (LPMMC) | Détails Fermer |
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Effective properties of condensate mixtures in the Gross-Pitaevskii regime. le mercredi 17 octobre 2018 à 11:00 |
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Résumé : Condensate mixtures consist of two or more different species of identical bosons which undergo Bose-Einstein condensation. In a recent joint work with Alessandro Michelangeli and Phan Thành Nam, we rigorously proved that, for mixtures in the Gross-Pitaevskii regime, the ground state energy is captured by the minimum of a suitable effective functional. We also studied the time-dependent behavior, and proved that the effective dynamics of mixtures is ruled by coupled non-linear Schroedinger equations, one for each species. I will devote the initial part of the talk to introducing the mathematical description of BEC and the main features of the Gross-Pitaevskii scaling limit. Then, prior to presenting our results, I will also recall some important known results on single-species. Liens :LPMMC |
| Volker Meden (RWTH Aachen) | Détails Fermer |
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Dynamical regimes of dissipative quantum systems le mercredi 10 octobre 2018 à 14:00 |
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Résumé : We reveal several distinct regimes of the relaxation dynamics of a small quantum system coupled to an environment within the plane of the dissipation strength and the reservoir temperature. This is achieved by discriminating between coherent dynamics with damped oscillatory behavior on all time scales, partially coherent behavior being nonmonotonic at intermediate times but monotonic at large ones, and purely monotonic incoherent decay. Surprisingly, elevated temperature can render the system “more coherent†by inducing a transition from the partially coherent to the coherent regime. We furthermore show that non-Markovian memory plays a prominent role in the time evolution after a quantum quench in such systems. This provides a refined view on the relaxation dynamics of open quantum systems. Liens : |
| Giancarlo Strinati (University of Camerino) | Détails Fermer |
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Gap equation with pairing correlations beyond mean field and its equivalence to a Hugenholtz-Pines condition for fermion pairs le vendredi 05 octobre 2018 à 11:00 |
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Liens : |
| Eckhard Krotscheck (University at Buffalo SUNY) | Détails Fermer |
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Structure and Dynamics of Quantum Fluids Confronting Theory and Experiments le vendredi 28 septembre 2018 à 11:00 |
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Résumé : The structure of quantum fluids is nowadays well understood from modern methods of quantum many-body theory, until recently the understanding of dynamic features has been much more limited. For that purpose, we have introduced the concept of dynamic multiparticle fluctuations i.e. processes which describe the dynamics of the short--ranged structure of the liquid. These processes turn out to be essential for the correct description of the dynamics of He-4 and He-3 in 3D. Carrying out the same types of calculations in 2D leads to somewhat surprising results which permit, in He-3, a decisive identification of the relevant physical effects responsible for the energetics of excitations, and may, if experimentally confirmed, illuminate the nature of the ``roton minimum'' in He-4. Practically simultaneously with the theoretical developments, new high precision neutron scattering measurements have been obtained of the group of Henri Godfrin in Grenoble which show the effects of mode-mode coupling with unprecedented accuracy. Theoretical predictions and experimental results are in very satisfactory agreement. Liens : |
| Doctorants et post-docs (LPMMC) | Détails Fermer |
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La nomadisation du parcours des docteurs le mercredi 26 septembre 2018 à 10:00 |
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Résumé : Nous aurons une discussion sur les différentes questions soulevées lors des journées hors-murs concernant la nomadisation des jeunes docteurs, les perspectives d'emploi etc. Liens :LPMMC |
| Pablo Rodriguez-Lopez | Détails Fermer |
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Quantum friction with Unruh-deWitt detectors le vendredi 21 septembre 2018 à 11:00 |
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Résumé : We revisit the atom-plate quantum friction and Casimir force with a full-relativistic formalism for atoms modelled as Unruh-deWitt detectors [1] in exited, relaxed and coherent superposition close to a plate [2]. We show that, for relative velocities close to c, the quantum friction diverges while the Casimir force is almost independent of the velocity. We are able to include the effect of the finite size of the detector, then we also obtain quantum friction when the detector is isolated but follows a non-inertial trajectory and we obtain a more realistic result for short distance interactions. Those studies open the venue to understand the role of non-local response in quantum friction. [1] E. Martin-Martinez and P. Rodriguez-Lopez. Relativistic quantum optics: The relativistic invariance of the light-matter interaction models. Phys. Rev. D 97, 105026 (2018) [2] P. Rodriguez-Lopez and E. Martin-Martinez. Casimir Forces and Quantum friction of finite-size atoms in relativistic trajectories. Accepted in Phys. Rev. A Liens : |
| Sergey Skipetrov (LPMMC) | Détails Fermer |
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Anderson localization of vector waves le mercredi 19 septembre 2018 à 11:00 |
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Résumé : Anderson localization was first discovered for electrons in disordered solids but later was shown to take place for various types of waves in disordered media. For three-dimensional (3D) disorder, it takes place only in a restricted band of frequencies, separated from the rest of the spectrum by mobility edges, and only when the disorder is strong enough. Our recent results indicate that the vector nature of waves (microwaves, light, elastic waves) used in the experiments on Anderson localization, plays an important role. In particular, the transverse electromagnetic waves cannot be localized by a random 3D arrangement of resonant point-like scatterers (atoms), whereas the elastic waves, which have a longitudinal component as well, can be localized in a way very similar to scalar waves. However, the localization of light can still be made possible by putting the atoms in a strong external magnetic field. We will present a unified view on Anderson localization and compute the localization phase diagrams and the critical parameters (mobility edges and critical exponents) of Anderson localization transitions for elastic waves and light scattered by atoms in a strong magnetic field. Despite the differences between these two systems, they turn out to belong to the same universality class. Liens :LPMMC |
| Lukas Sieberer | Détails Fermer |
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Order by anisotropy in two-dimensional driven-open systems le vendredi 14 septembre 2018 à 11:00 |
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Résumé : The spatial and temporal order of two-dimensional systems with a continuous U(1) symmetry is determined by the dynamics of vortices. At low temperatures, vortices of opposite charge form tightly bound pairs, while they are free to roam and destroy order as the temperature is increased. Interestingly, driving the system out of equilibrium alters the interaction of vortices in a drastic way: Instead of being long-ranged and thus capable of holding together pairs of vortices and anti-vortices in the ordered phase, out of equilibrium the interaction becomes screened, and defects proliferate. Here, we show that the structure of defects and their interaction can equally dramatically be modified by the breaking of rotational symmetry. For sufficiently strong spatial anisotropy, the force that binds pairs of defects can even be enhanced up to parametrically large scales. As a consequence, the vortex-unbinding crossover in such finite-size systems exhibits peculiar universal behavior. In the thermodynamic limit, we argue that the modified structure of defects renders a stable ordered phase possible. These results, which we obtain by analyzing the compact anisotropic Kardar-Parisi-Zhang (caKPZ) equation, are relevant for a wide variety of physical systems, ranging from strongly coupled light-matter quantum systems such as exciton-polaritons, to recently proposed classical time crystals. Liens : |
| Doron Cohen (Ben Gurion University) | Détails Fermer |
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Quantum thermalization and STIRAP through chaos in Bose-Hubbard circuits le mercredi 12 septembre 2018 à 11:00 |
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Résumé : We clarify the role of "quantum chaos" in the analysis of Bose-Hubbard
circuits. Specifically we address themes that are related to
thermalization and localization [1], quasi-static sweep processes [2],
and meta-stability of condensates [3].
References[1] A. Dey, D. Cohen, A. Vardi, arXiv:1805.05165[2] C. Khripkov, A. Vardi, D. Cohen, PRE 97, 022127 (2018) [3] G. Arwas, D. Cohen (in preparation). |
| Enrico Compagno (LPMMC) | Détails Fermer |
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Two-boson correlations in three weakly coupled Bose-Einstein condensates le mercredi 05 septembre 2018 à 10:00 |
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Liens :LPMMC |
| Konstantin Bliokh (RIKEN, Japan) | Détails Fermer |
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Optical momentum and angular momentum in complex media le vendredi 06 juillet 2018 à 11:00 |
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Liens : |
| Tobias Haug (Center for quantum technologies) | Détails Fermer |
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Andreev-reflection and Aharonov-Bohm dynamics in atomtronic circuits le mercredi 04 juillet 2018 à 11:00 |
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| Alioscia Hamma (University of Massachusetts Boston) | Détails Fermer |
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Entanglement Complexity and the Emergence of Irreversibility in Quantum Mechanics le vendredi 29 juin 2018 à 11:00 |
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Résumé : The onset of irreversibility in physics is one of the great questions at the heart of statistical mechanics. The second principle of thermodynamics in essence states that spontaneous processes happen in one direction. In classical physics irreversibility can only happen with some seed of randomness or coarse graining and topological mixing. Since coarse graining and the counting of micro states is arbitrary in classical physics, firmer grounds for statistical mechanics must be found in the quantum domain. At a first glance the quantum case looks even harder. In a closed system evolution is unitary, and therefore the entropy of a quantum state cannot increase. Moreover, unitary evolution is always reversible, so irreversibility is strictly speaking impossible. In classical mechanics irreversibility is due to chaos, that is, high sensitivity to initial conditions. But in quantum mechanics, unitarity implies that slightly different initial conditions do not evolve into highly different states. In this talk we take seriously the idea that the defining feature of quantum mechanics is entanglement. As such, irreversibility must be a consequence of entanglement. As we shall see, it is not the amount of entanglement per se that is important, but its complexity. We show that complexity of entanglement classifies the dynamical behavior of a isolated quantum many-body system and determines its irreversibility and the approach to thermalization. Liens : |
| Piero Naldesi (LPMMC) | Détails Fermer |
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Raise and fall (and spin) of a bright soliton in an optical lattice le mercredi 27 juin 2018 à 11:00 |
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Liens :LPMMC |
| Andrew Daley | Détails Fermer |
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Controlled dissipation and dynamics in quantum simulators le vendredi 22 juin 2018 à 11:00 |
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Résumé : Over the past few years, several experimental platforms from Atomic, Molecular and Optical (AMO) physics have begun to be used as quantum simulators. These are devices where we have excellent microscopic understanding and control – allowing us to write down microscopic models under well-controlled approximations, and to adjust the parameters of these models to explore a wide range of phenomena arising from many-body physics. An important aspect of these systems that is often not widely discussed is that we also have excellent microscopic understanding of the dissipative dynamics of these systems, and means to engineer this dissipation (e.g., via controlled light scattering, or introduction of a reservoir gas). This provides new possibilities to observe the effects of dissipation on many-body dynamics, and also new tools to produce interesting many-body quantum states. I will discuss this, with examples from our recent theoretical work on dissipative engineering of spin-entangled states, and exploration of light scattering and its effects on the dynamics and decoherence of many-body states in optical lattices. Liens : |
| Stagiaires M2 (LPMMC) | Détails Fermer |
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(titre non communiqué) le mercredi 20 juin 2018 à 11:00 |
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Résumé : - Anastasia - Frederick - candidat de Sergey - Théotime Liens :LPMMC |
| Tin Sulejmanpasic (ENS) | Détails Fermer |
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Fractionalization between the vacua: from QCD to quantum magnetism le vendredi 15 juin 2018 à 11:00 |
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Résumé : Quantum Chromodynamics (QCD) -- the theory of strong nuclear forces -- has baffled the physics community and remains one of the poorly understood parts of the standard model. Its quintessential property: the confinement of quarks into protons, neutrons and mesons, while verified both experimentally and numerically, remains an elusive theoretical problem. The various cousins of QCD are however possible to understand to varying degrees and precision. In some of these theories the vacuum state is degenerate, and hence allows for domain walls -- a surface excitation which interpolates between two vacua of the theory. These domain walls have a remarkable property that quarks become liberated on them, and the domain wall excitation spectrum is very different from that of the bulk. Such QCD cousins are, unfortunately, not the physical theory, and they do not occur in nature. QCD however has another unlikely cousin: the Valence Bond Solid (VBS) state of the quantum anti-ferromagnet, where spin 1/2 excitations (or spinons) are bound into spin 1 excitations by a mechanism very similar to confinement of quarks. Perhaps surprisingly the low energy theory describing the behavior of the VBS phase is virtually identical to its QCD cousins under certain conditions. Further the VBS phase may have multiple vacua, and thus support domain walls, which in turn support liberated spinon excitations absent in the bulk. This has been verified numerically in the so-called J-Q model. These domain wall modes can in fact be seen as edge modes akin to those of the symmetry protected topological state. A multidisciplinary effort is slowly emerging to understand such phenomena, from the theoretical aspects of fundamental and condensed matter physics, to the numerical efforts in trying to understand QCD and quantum magnets. Liens : |
| Jacopo (LPMMC) | Détails Fermer |
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Dynamical properties of impenetrable bosons in optical lattices le mercredi 13 juin 2018 à 11:00 |
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Résumé : The study of strongly correlated quantum systems is one of the most interesting and intriguing research field in physics. The framework of ultracold gases in optical lattices allows to explore equilibrium and non-equilibrium properties of such systems. One example is the Tonks-Girardeau (TG) gas, in which the infinite repulsive delta- like interaction mimics the Pauli exclusion principle and is reflected into the well known mapping to non-interacting fermions. While local quantities are identical to the fermionic case, all non local ones, like correlations or momentum distribution, are significally different. We develop a powerful method to study the spectral function of the TG gas by using only single particle orbitals, and apply it to inspect the behaviour of ultracold gases in a periodic lattice with hard-wall confining. Moreover, the efficient implementation of the one body green’s functions provide an instrument to investigate energy and mass transport in periodic media and quasicrystals via the scheme of non equilibrium green’s functions. Liens :LPMMC |
| Vincent Michal (Qutech - TU Delft) | Détails Fermer |
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Interaction without back-action in the context of quantum manipulation le vendredi 08 juin 2018 à 11:00 |
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Résumé : We study the interaction between two quantum systems (A and B) that is mediated by their common linear environment. If the environment is out of equilibrium the resulting interaction violates Onsager relations and cannot be described by a Hamiltonian. In simple terms the action of system A on system B does not necessarily produce a back action. We derive general quantum equations describing the situation and analyze in details their classical correspondence. Changing the properties of the environment one can easily change and engineer the resulting interaction. It is tempting to use this for quantum manipulation of the systems. However the resulting quantum gate is not always unitary and may induce a loss of quantum coherence. For a relevant example we consider systems A and B to be spins of arbitrary values and arrange the interaction to realize an analogue of the two-qubit CNOT gate. The direction of spin A controls the rotation of spin B while spin A is not rotated experiencing no back-action from spin B. We solve the quantum dynamics equations and analyze the purity of the resulting density matrix. The resulting purity essentially depends on the initial states of the systems. We attempt to find a universal characteristics of the purity optimizing it for the worst choice of initial states. For both spins sA=sB=1/2, the optimized purity is bounded by 1/2 irrespective of the details of the gate. We also study in detail the semiclassical limit of large spins. In this case the optimized purity is bounded by (1 et Ï€/2)-1 ≈ 0.39. This is much better than the typical purity of a large spin state ∼ s-1. We conclude that although the quantum manipulation without back-action inevitably causes decoherence of the quantum states the actual purity of the resulting state can be optimized and made relatively high. |
| Nicolas Victorin (LPMMC) | Détails Fermer |
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Excitations and correlations in Bose-Hubbard coupled rings le mercredi 06 juin 2018 à 11:00 |
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Liens :LPMMC |
| CPTGA 1er juin (Café (Florida State University) | Détails Fermer |
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Quantum Critical Behavior at the Mott Point: the Status Quo le vendredi 1er juin 2018 à 11:00 |
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Liens : |
| Keith Gilmore (European Synchrotron Radiation Facility, Grenoble) | Détails Fermer |
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Reproducing dynamical excitations observed in resonant inelastic X-ray scattering through Bethe-Salpeter calculations le vendredi 25 mai 2018 à 11:00 |
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Résumé : Resonant inelastic X-ray scattering (RIXS) is a relatively new technique for probing low energy excitations in materials. In addition to traditional techniques, such as angle resolved photoemission, it has become an important, high precision characterization tool of strongly correlated electron materials. To calculate RIXS, and related core and valence level spectra, we solve the Bethe-Salpeter equation (BSE) based on a self-energy corrected density functional theory electronic structure. I outline our implementation of the BSE and use SrVO3 for demonstration. Non fluorescence features in RIXS arise from a dynamic response of the system to the intermediate state perturbation. Since the Bethe-Salpeter equation is typically reduced to the static limit in practice, these dynamic excitations are generally not reproduced. To include interactions beyond the static BSE I introduce the cumulant expansion. Spectral functions derived from a GW self-energy are typically inadequate when the dressed Green’s function is built via the Dyson equation. With the same GW self-energy, a superior Green’s function and spectral function, implicitly including vertex corrections, is obtained through the cumulant expansion. I consider application of cumulant spectral functions to photoemission, photoabsorption, and X-ray scattering. Lastly, vibronic coupling has important impacts on these spectra. I show how to calculation the phonon contribution to photoemission, absorption and scattering with a vibronic cumulant. Liens : |
| Jordan Hervy (ILL, LPMMC) | Détails Fermer |
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(titre non communiqué) le mercredi 16 mai 2018 à 13:30 |
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Liens :ILL, LPMMC |
| Paolo Politi (Institute for Complex Systems, CNR - Florence) | Détails Fermer |
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Conservation laws and nonequilibrium dynamics le vendredi 04 mai 2018 à 11:00 |
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Résumé : One of the simplest examples of nonequilibrium dynamics varying in the presence of a conservation law is the coarsening process of the kinetic Ising model, whose exponent changes from 1/2 to 1/3 if the magnetization is conserved. We start showing that this difference may be strongly enhanced by increasing the interaction range of the ferromagnetic coupling, which determines a speed-up in nonconserved coarsening while it almost freezes conserved coarsening [1]. Conserved quantities are also relevant to obtain a condensation-like transition in non interacting systems: we will discuss the relaxation process of a simple model, which may be considered as a rough approximation of the Discrete Nonlinear Schroedinger Equation [2], whose dynamics is actually much, much slower. [1] Federico Corberi, Eugenio Lippiello and Paolo Politi Effective mobility and diffusivity in coarsening processes EPL 119, 26005 (2017) [2] Stefano Iubini, Antonio Politi and Paolo Politi Relaxation and coarsening of weakly-interacting breathers in a simplified DNLS chain J. Stat. Mech.: Theory and Experiments, 073201 (2017) Liens : |
| Jordan Hervy (LPMMC) Annulé | Détails Fermer |
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(titre non communiqué) le mercredi 25 avril 2018 à 11:00 |
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Liens :LPMMC |
| Louk Rademaker (Perimeter Institute) | Détails Fermer |
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Quenching the Kitaev honeycomb model le vendredi 20 avril 2018 à 11:00 |
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Résumé : The Kitaev honeycomb model is one of the few exactly solvable models with a spin liquid ground state. Here I will discuss the dynamics of an initial antiferromagnetic state time evolved with the Kitaev model. I find a dynamic crossover to a valence bond solid. When the spin interactions are anisotropic, an exponentially long prethermalized regime appears. Reference: arXiv:1710.09761 Liens : |
| Markus Holtzmann (LPMMC) | Détails Fermer |
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(titre non communiqué) le mercredi 04 avril 2018 à 11:00 |
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Liens :LPMMC |
| Enderalp Yakaboylu (Institute of Science and Technology Austria, Klosterneuburg, Austria) | Détails Fermer |
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Emergence of non-Abelian magnetic monopoles and Anyonic statistics in quantum impurity problems le vendredi 23 mars 2018 à 11:00 |
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Résumé : By virtue of emergent gauge fields in quantum impurity problems, we demonstrate that in experimentally realized regime the angulon, a quantum rotor dressed by bosonic excitations, can be seen as a point charge on a two-sphere interacting with a gauge field of non-Abelian monopole. We find a topological transition associated with making the monopole Abelian, which takes place in the vicinity of the previously reported angulon instabilities. Furthermore, we show that identical impurities interacting with a two dimensional many-particle environment obey anyonic statistics. In particular, we find that due to the many-body interactions between impurities and the bath, each of the impurities can be viewed as a flux-tube-charged-particle composite described by fractional statistics. This amounts to a novel configuration with emerging anyons, which is fundamentally different from the previously studied fractional quantum Hall and Kitaev model settings. Liens : |
| Peter Schuck (LPMMC) | Détails Fermer |
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Pairing in inhomogeneous systems and quantum clusters : cold atoms in traps, metallic grains, atomic nuclei, etc. le mercredi 21 mars 2018 à 11:00 |
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Liens :LPMMC |
| Juan Polo Gomez (LPMMC) | Détails Fermer |
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Damping of Josephson oscillations in strongly correlated one-dimensional atomic gases le mercredi 14 mars 2018 à 11:00 |
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Résumé : We study the Josephson oscillations of two strongly correlated one-dimensional bosonic clouds separated by a localized barrier. Using a quantum-Langevin approach and the exact Tonks-Girardeau solution in the impenetrable-boson limit, we determine the dynamical evolution of the particle-number imbalance, displaying an effective damping of the Josephson oscillations which depends on barrier height, interaction strength and temperature. We show that the damping originates from the quantum and thermal fluctuations intrinsically present in the strongly correlated gas. Thanks to the density-phase duality of the model, the same results apply to particle-current oscillations in a one-dimensional ring where a weak barrier couples different angular momentum states. Liens :LPMMC |
| Rob Whitney (LPMMC) | Détails Fermer |
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Laws of thermodynamics and fluctuation theorems for quantum machines le vendredi 09 mars 2018 à 11:00 |
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Résumé : Consider quantum dots (or other nanostructures) which can convert a heat flow into electrical power, or can use power to move heat from a cold reservoir to a hot one. Do such systems obey the same laws of thermodynamics as macroscopic heat engines? How does one get the second law of thermodynamics when the Schrodinger equation is invariant under time-reversal? What approximations can one make on the dynamics without unphysical violations of the laws of thermodynamics? I consider these questions in the context of a real-time Keldysh theory of transport for a nanostructure coupled to reservoirs of electrons and phonons, with particular interest in the difficult case of strong system-reservoir coupling. I explicitly formulate how our lack of information leads to entropy production with what I call a "no Maxwell demons" assumption. I then show that a simple microscopic symmetry in the Feynman diagrams enables one to show that the first and second law of thermodynamics are obeyed on average, but that fluctuations violate them. I show that these fluctuations obey a variety of fluctuation theorems (Jarzynski equality, Crooks equation, etc). Finally I mention the well-known approximations that satisfy this symmetry, and thus will not lead to unphysical thermodynamics. Liens : |
| Masahiro 07 mars (LPMMC) | Détails Fermer |
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Adiabatic pumping induced by reservoir parameter driving in a quantum dot system. le mercredi 07 mars 2018 à 11:00 |
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Liens :LPMMC |
| Paolo Politi (Istituto dei Sistemi Complessi, CNR, Florence) Annulé | Détails Fermer |
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Conservation laws and nonequilibrium dynamics le vendredi 02 mars 2018 à 11:00 |
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Annulé
Liens : |
| CPTGA 14 février (Café (Okinawa Institute of Science and Technology Graduate University) | Détails Fermer |
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Quantum dynamics in the presence of interactions le mercredi 14 février 2018 à 11:00 |
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Résumé : Interactions between atoms often introduce large amounts of complexity into many-particle systems, but they can also lead to new and interesting physical regimes. In this presentation I will discuss several examples of interacting ultra cold atomic systems, where tuneable interactions allow to access new dynamical situations or create new control techniques. The first example will be taken from our efforts towards a full set of techniques to coherently control the external state of small samples of ultracold atoms using spatial adiabatic passage, and in the second an increased interaction in a multicomponent BEC system is shown to lead to emergence of classical behaviour in a fully quantum mechanical system. Liens : |
| Andrej Mesaros (LPS - Orsay) | Détails Fermer |
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Fractionalized particles on defects in topological insulators and superconductors le vendredi 09 février 2018 à 11:00 |
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Résumé : Recent experiments on one-dimensional and two-dimensional materials have been very successful in realizing topological states of electrons, with one of ultimate goals being the creation of emergent Majorana particles. Such emergent particles can have fractional charge, spin, and/or quantum statistics, so they are interesting both fundamentally and for quantum computing applications, but remain hard to realize. We will discuss the use of topological defects as a way to realize fractional particles. By combining analytic and numerical approaches we predict Majorana particles, having favorable energetic properties, in certain vortices of two-dimensional superconductors. Our predictions fully explain puzzling features of recent experiments at INSP Jussieu. We will also focus on lattice dislocations in topological insulators as realizations of more complex fractional particles. General properties of fractionalized particles and open questions will also be discussed. Liens : |
| Jacopo Settino (Université de Calabre) | Détails Fermer |
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Static and dynamical properties of ultracold gas in a periodic and incommensurate potential le mercredi 07 février 2018 à 11:00 |
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Résumé : We explore static and dynamical properties of fermionic and hardcore bosonic cold atomic gas trapped by the combination of two potentials (bichromatic lattice) with incommensurate periods. Firstly we study the effect of the metal to insulator transition, and the presence of a mobility edge, into many-body measurable quantities, such as the momentum distribution. Then we study the long time dynamics of the gas, subject to a quantum quench, by looking at both single particle and global quantities, namely the single particle Green's function and the Loschmidt echo. In the case of a periodic lattice we show that the asymptotic dynamics manifests the Anderson Orthogonality Catastrophe (AOC) and we find a general analytic expression for the power law exponent which is then compared with our numerical results. On the other hand in the case of an incommensurate potential, which shows a mtal-to-insulator transition, we observe the suppression of the AOC and an anomalous spreading of correlations as the transition point is approached. We discuss these results from the point of view of the nature of the single particle energy spectrum and show that these anomalous features come from the particular geometry of the system. Liens : |
| Jerome Dubail (Institut Jean Lamour, Université de Lorraine) | Détails Fermer |
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Hydrodynamics of 1d bosons with delta repulsion le vendredi 02 février 2018 à 11:00 |
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Résumé : Describing and understanding the motion of quantum gases out of equilibrium is a tremendous challenge for theorists. In 2006, the groundbreaking Quantum Newton Cradle experiment [1], where it was observed that two 1d clouds of cold atoms bounce against each other indefinitely without relaxation, provided impetus for many developments on the effects of low dimensionality in out-of-equilibrium quantum physics. But it is only thanks to the breakthrough of « Generalized HydroDynamics (GHD) » in 2016 [2] that one now possesses the adequate tools for an effective large-scale description of that experiment [3]. The purpose of this talk will be to give an introduction to those recent theoretical advances. Refs: [1] Kinoshita, Wenger and Weiss, Nature 440, 900, 2006 [2] Castro-Alvaredo, Doyon and Yoshimura, PRX 6, 041065, 2016 and Bertini, Collura, de Nardis and Fagotti, PRL 117, 207201, 2016 [3] Caux, Doyon, Dubail, Konik, Yoshimura, arXiv:1711.00873 Liens : |
| Gianluca Catelani (Forschungszentrum Juelich) | Détails Fermer |
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Measuring and controlling quasiparticles in superconducting qubits le mercredi 24 janvier 2018 à 11:00 |
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Liens : |
| Enrico Compagno (LPMMC) | Détails Fermer |
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Lattice Based Low Control Quantum Technology le mercredi 17 janvier 2018 à 11:00 |
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Liens :LPMMC |
| Juan Polo (LPMMC) | Détails Fermer |
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Solitons and C2-kaleidoscope le vendredi 12 janvier 2018 à 11:00 |
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Liens :LPMMC |
| MISSING | Détails Fermer |
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A study on the topological nature of Landau levels in a 2D muffin-tin potential lattice le mardi 09 janvier 2018 à 14:30 |
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Liens : |
| Etienne Jussiau (LPMMC) | Détails Fermer |
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(titre non communiqué) le jeudi 21 décembre 2017 à 11:00 |
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Liens :LPMMC |
| Cécile Répellin (MIT) | Détails Fermer |
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Stability of the spin-1/2 kagome ground state with breathing anisotropy le lundi 18 décembre 2017 à 11:00 |
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Résumé : Quantum spin liquids (QSLs) are strongly correlated phases which cannot be characterized by a spontaneous symmetry breaking at zero temperature. Their exotic features (such as fractionalized excitations and topological properties) and the prospect of realizing them in frustrated magnets have aroused a lot of interest. The kagome lattice antiferromagnet is one the main model candidates which may realize a QSL. It poses a challenge to theorists and experimentalists alike: materials such as herbertsmithite can be approximated by this model but additional terms and disorder may change the nature of the ground state entirely. On the theoretical front, the ground state of the ideal model is generally admitted to be a QSL whose precise nature remains one of the most debated questions of the field, the (gapped) topological Z2 spin liquid and (gapless) dirac spin liquid being two of the strongest candidates. We study the spin-1/2 breathing (or trimerized) kagome lattice. In this variation of the kagome Heisenberg antiferromagnet (which appears in a recently synthesized vanadium compound), the spins belonging to upward and downward facing triangles have different coupling strengths. Beyond the experimental motivation, connecting the ideal kagome ground state to its fully trimerized counterpart may bring important insight into the nature of the kagome ground state, as strong coupling approaches have suggested the importance of the trimerized model as an effective model capturing most low energy degrees of freedom. Using DMRG and exact diagonalizations, we show the large stability of the kagome ground state upon introducing the breathing anisotropy. Exploration of the entanglement properties of the ground state confirm this picture, and reveal the persistence of signatures of Dirac excitations even for relatively large breathing anisotropy. Finally, we closely examine the limit of strong breathing anisotropy and find indications of a transition to a nematic phase. Liens : |
| Nicolas Victorin (LPMMC) | Détails Fermer |
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Bosonic double lattice ring under a gauge field le jeudi 14 décembre 2017 à 10:30 |
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Liens :LPMMC |
| Leonardo Mazza (ENS) | Détails Fermer |
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Majorana fermions in particle-conserving settings le vendredi 08 décembre 2017 à 11:00 |
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Résumé : The paradigmatic condensed-matter models where zero-energy localized Majorana fermions have been studied so far have the distinguishing feature of not conserving the number of fermions. Moreover, the accepted definition of Majorana fermion naturally belongs to this scenario. Is it possible to discuss Majorana fermions in canonical particle-conserving settings? In this seminar I will present several exact and numerical results on Majorana fermions in particle-conserving scenarios. I will start from the discussion of a model for bosons and fermions where, in a proper limit, the physics of the celebrated Kitaev chain appears. I will continue by presenting exact results on Majorana fermions in ladder models where the two legs of the system can only exchange pair of particles. Finally, I will comment on the possibility of making experiments with Majorana fermions in particle-conserving settings. References: Iemini, LM, Rossini, Fazio and Diehl, PRL 115, 156402 (2015) Iemini, LM, Fallani, Zoller, Fazio, Dalmonte, PRL 118, 200404 (2017) Liens : |
| Piero Naldesi (LPMMC) | Détails Fermer |
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Soliton interferometry in a ring le jeudi 07 décembre 2017 à 11:00 |
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Liens :LPMMC |
| Michele Fillipone | Détails Fermer |
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The Interacting Mesoscopic Capacitor Out of Equilibrium le vendredi 1er décembre 2017 à 11:00 |
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Résumé : The mesoscopic capacitor has revealed an efficient quantum device to achieve the triggered emission of single coherent electrons in solid state systems. The role of electron-electron interactions in these systems strongly driven out-of-equilibrium still requires clarification. We consider the full nonequilibrium response of a mesoscopic capacitor in the large transparency limit, exactly solving a model with electron-electron interactions appropriate for a cavity. For a cavity coupled to the electron reservoir via an ideal point contact, we show that the response to any time-dependent gate voltage Vg(t) is strictly linear in Vg. We analyze the charge and current response to a sudden gate voltage shift, and find that this response is not captured by a simple circuit analogy. In particular, in the limit of strong interactions a sudden change in the gate voltage leads to the emission of a sequence of multiple charge pulses, the width and separation of which are controlled by the charge-relaxation time Ï„c=h Cg/e2 and the time of flight Ï„f. Our results are compared with recent noise measurements in Hong-Ou-Mandel experiments (Freulon et al. Nat. Comm. 6, 6854 (2015)). Our approach justifies the presence of an unexplained dip in the noise as a function of the time delay of activation of the two sources and highlights the unexpected importance of interaction effects in the dynamics of quantum cavities in these experiments. We also consider the effect of a finite reflection amplitude in the point contact, which leads to nonlinear-in-gate-voltage corrections to the charge and current response. ReferencePhys. Rev. B 96, 085429 (2017) Liens : |
| Aleksandr Svetogorov (LPMMC) | Détails Fermer |
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Coherent quantum phase-slips in one-dimensional inhomogeneous superconductors le jeudi 30 novembre 2017 à 11:00 |
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Liens :LPMMC |
| Artur Slobodeniuk | Détails Fermer |
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Fine structure of multilayer TMDC: when more is different le jeudi 23 novembre 2017 à 11:00 |
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Liens : |
| Jiri Minar (University of Nottingham, School of Physics and Astronomy) | Détails Fermer |
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Localization phenomena and topological properties of atomic lattice gases with long-range interactions le vendredi 17 novembre 2017 à 11:00 |
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Liens : |
| Pierre Nataf (ETH Zürich) | Détails Fermer |
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Numerical methods to investigate Heisenberg SU(N) lattice models. le jeudi 16 novembre 2017 à 11:00 |
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Résumé : Systems of multicolor fermions have recently raised considerable interest due to the pos-
sibility to experimentally study those systems on optical lattices with ultracold atoms [1].
To describe the Mott insulating phase of N-colors fermions, one can start with the SU(N)
Heisenberg Hamiltonian. In the case of one particule per site, the SU(N) Heisenberg Hamiltonian takes the form of a Quantum permutation Hamiltonian H = J∑*lt;i,j>Pij , where the
transposition operator Pij exchanges two colors on neighboring sites.
We have developped a method[2] to implement the SU(N) symmetry in an Exact Diagonalization algorithm. In particular, the method enables one to diagonalize the Hamiltonian
directly in the irreducibe representations of SU(N), thanks to the use of standard Young
tableaux[3], which are shown to form a very convenient basis to diagonalize the problem. It
allowed us to prove that the ground state of the Heisenberg SU(5) model on the square lat-
tice is long range color ordered [2] and it provided evidence that the phase of the Heisenberg
SU(6) model on the Honeycomb lattice is a plaquette phase [4]. Finally, SU(N) chiral phases
on the triangular lattice with artificial gauge fields are also investigated and characterized
through ED[5]. Liens :Pierre NatafETH Zürich |
| Itai Arad (Physics Department, Technicon) | Détails Fermer |
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Efficient representation of many-body ground states le vendredi 10 novembre 2017 à 11:00 |
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Résumé : Quantum Hamiltonian Complexity is a branch of quantum information which looks at quantum many-body Hamiltonians, the backbone of condensed-matter physics, through the lenses of quantum information and computational complexity. In this talk I will demonstrate this unique perspective by asking to what extent ground states of quantum spin systems are quantum or classical. I will study this question by defining "classical states" as those which can be well-approximated using only a polynomial number of degrees of freedom, despite the fact that they live in an exponentially large Hilbert space. I will explain why this problem was essentially solved for the case of one dimensional spin chains, and why it is still wide open for systems in higher dimensions. I will also explain how it is related to the existence of area laws and tensor-networks description. Finally, I will present some recent progress on this question for a class of frustrated 2D systems. Liens : |
| CPTGA 06 novembre (University of Alberta, Edmonton) | Détails Fermer |
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The fate of microemulsions in two-dimensional systems: yes, sometimes numbers matter le lundi 06 novembre 2017 à 14:00 |
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Résumé : Two-dimensional systems of particles interacting via a purely repulsive potential, decaying at large distances as the cubic inverse power of the distance, have been the subject of much theoretical and experimental investigation. On general grounds, an ordinary fluid to crystal phase transition should take place at zero temperature, on increasing the density. However, an elegant argument was provided a decade ago [1] to the effect that no conventional first order phase transition could occur, as ordinary coexistence of fluid and crystal phases separated by a microscopic interface would be energetically unstable at low temperature. For, the system could lower its energy by forming a so-called microemulsion, namely a novel phase of matter featuring large solid clusters ("bubbles") floating in the fluid. This intriguing prediction could in principle be observed experimentally in assemblies of cold dipolar atoms, but also in excitonic systems in semiconductor quantum wells. Failure to observe this scenario experimentally or in numerical simulation prompted a reexamination of the original argument; as it turns out, the characteristic size of the bubbles in the micro motion crucially depends on specific features of the phase transition, such as melting and freezing densities, as well as on the energy per unit length of a macroscopic interface separating the two phases. No reliable estimate was available of any of the three quantities until recently, when the first first principle calculation was completed. I shall describe it in this talk, and argue that based on its results the micro emulsion scenario is of "academic" interest only, due to the astronomically large length scales involved. [1] B. Spivak and S. Kivelson, Phys. Rev. B 70, 155114 (2004)​ Liens : |
| Matthieu Vanicat (Department of Theoretical Physics, Ljubljana) | Détails Fermer |
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Matrix ansatz in integrable non-equilibrium models le jeudi 02 novembre 2017 à 11:00 |
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Résumé : I will present new examples of exactly solvable exclusion processes. They are models of particles in interaction on a one dimensional lattice with L sites. The particles are evolving randomly on the lattice following simple stochastic rules. The lattice is connected at its extremities to particle reservoirs with different densities which drive the system out-of-equilibrium. I will explain how to compute exactly the stationary distribution (which does not obey a Boltzmann statistics) in a matrix product form. This will allow us to compute analytically physical quantities such as particle current and correlation functions. We will also be able to make connection with an hydrodynamic description: the Macroscopic Fluctuation Theory. |
| Jordi Boronat (Departament de Fisica, Universitat Politecnica de Catalunya, Barcelona, Spain) | Détails Fermer |
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Ultradilute drops of bosons le vendredi 20 octobre 2017 à 11:00 |
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Résumé : Strongly interacting systems of dipolar bosons in three dimensions confined by harmonic traps are analyzed using the exact path integral ground-state Monte Carlo method. By adding a repulsive two-body potential, we find a narrow window of interaction parameters leading to stable ground-state configurations of droplets in a crystalline arrangement. We find that this effect is entirely due to the interaction present in the Hamiltonian without resorting to additional stabilizing mechanisms or specific three-body forces. In a different context, we will show preliminary results on the formation of dilute liquid drops in Bose mixtures with interspecies attraction. Liens : |
| Jose Maria Escalante Fernandez (LPMMC) | Détails Fermer |
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Anderson localization in classical waves : the role played by its vector character le jeudi 19 octobre 2017 à 11:00 |
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Liens :LPMMC |
| Maxim Olshanyi (University of Massachusetts Boston, Boston, USA) | Détails Fermer |
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Scattering of a Gross-Pitaevskii breather off a barrier: the Inverse Scattering Transform made tangible le vendredi 13 octobre 2017 à 11:00 |
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Résumé : A key observable signature of integrability---of the existence of infinitely many "higher" conservation laws---in a system supporting solitons is the fact that a collision between solitons does not change their shape or size. But then, if solitons meet on top of a strong integrability-breaking barrier, one would expect the solitons to undergo some process consistent with energy conservation but not with higher conservation laws, such as the larger soliton cannibalizing the smaller one. However, here we show that when a strongly-coupled "breather" of the integrable nonlinear Schrodinger equation is scattered off a strong barrier, the solitons constituting the breather separate but survive the collision: as we launch a breather with a fixed impact speed at barriers of lower and lower height, at first all constituent solitons are fully reflected, then, at a critical barrier height, the smallest soliton gets to be fully transmitted, while the other ones are still fully reflected. This persists as the barrier is lowered some more until, at another critical height, the second smallest soliton begins to be fully transmitted as well, etc., resulting in a staircase-like transmission plot, with _quantized_ plateaus. We show how this effect makes tangible the _inverse scattering transform_: the powerful, but otherwise physically opaque mathematical formalism for solving completely integrable partial differential equations. Supported by the NSF, ONR, and US-Israel BSF. In collaboration with V. Dunjko. Liens : |
| Sathishkumar Rangaswamy Kuppuswamy et | Détails Fermer |
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Satish : Modelling of Exciton Binding Energy in Semiconductor Nanocrystals et Achille: Dielectric Properties of Luttinger Semimetals le jeudi 12 octobre 2017 à 13:30 |
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Liens : |
| MISSING | Détails Fermer |
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BKT phase transition in 2D polariton condensates le jeudi 12 octobre 2017 à 11:00 |
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Résumé : Polariton condensation has been observed in many different systems, ranging from standard inorganic 2D microcavities and 1D wires to 0D confined systems. The coherence build up process behind the condensate formation has been widely studied but the presence of an exciton reservoir and the unavoidable effects of the finite size of the excitation area are detrimental and in some cases have been heavily underestimated. Thanks to a sample with very long polariton lifetime and with a marked spatial homogeneity it is possible to generate an extended condensed state outside of the laser spot. Here we show the fascinating exhibition of an equilibrium Berezinskii-Kosterlitz-Thouless (BKT) phase, despite the intrinsic dissipation character of polariton quasi-particles, characterised by a power-law coherence decay both in time and space domain. Such a combined observation opens the doors to the study of the excitations spectrum of driven-dissipative condensates with high energy resolution. Liens : |
| CPTGA 06 octobre (Café (LPTMS, Orsay) | Détails Fermer |
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Quantum coherence in bilayer graphene structures le vendredi 06 octobre 2017 à 11:00 |
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