• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : In the past decade superconductor-semiconductor hybrids have been studied intensively, with significant efforts towards studying Majorana bound states (MBSs). In this talk I will discuss a relatively new approach to realize MBSs using quantum dot-superconductor hybrids. I will focus on how MBSs can be systematically and reliably engineered in a two-dimensional electron gas by tuning the relative strengths of the different kinds of couplings between the quantum dots [1,2]. Ref.: [1] Triplet correlations in Cooper pair splitters realized in a two-dimensional electron gas; https://www.nature.com/articles/s41467-023-40551-z [2] Engineering Majorana bound states in coupled quantum dots in a two-dimensional electron gas; https://arxiv.org/abs/2311.03208

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle de Conférence du LNCMI

Résumé :

Triangular lattices are uncomfortable stages for interacting electrons, however, which bring about emergent states. In half-filled band systems, antiferromagnetically interacting spins are strongly frustrated and may exhibit a quantum spin liquid (QSL). In quarter-filled band systems, Coulomb interacting electrons fail to form a Wigner crystal on a triangular lattice but may freeze into a charge glass (CG) state, which even may quantum　melt. The organic k-ET salts and q-ET salts are good model systems for the former and latter subjects, respectively. I present our updated results on these two issues.

For the issue of QSL, I summarize the present experimental status on the QSL candidate, k-(ET)₂Cu₂(CN)₃, and also show unconventional properties (non-Fermi liquidity, quantum criticality, BEC-like superconductivity) in the doped QSL candidate, k-(ET)₄Hg_2.89Br₈. For the issue of CG, I show quantum-classical dual properties of glasses exhibited by a series of q-ET₂X.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : We study the effect of quantum fluctuations on the parametric instability of a degenerate parametric oscillator at threshold. Relying on a weak nonlinearity, we identify an (effective) universal Liouvillian consistent with the symmetries of the system that captures the slow (long-time) dynamics. We find that all cumulants exhibit universal power-law scaling with the nonlinearity, with the Fano factor showing a maximum near the threshold. For a voltage biased Josephson junction, the method of third quantization method is used to identify the slow modes and to derive the effective dynamics by adiabatic elimination of the fast modes. In this way, the parameters entering the effective model can be linked to the microscopic parameters of the experimental platform. Our findings offer insights into the oscillator\'s behavior and provide a foundation for understanding and predicting the parametric instability at threshold.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Beyond their applications in quantum computing, superconducting qubits are a powerful platform to probe various quantum phenomena in the context of hybrid quantum systems [1]. However, most of them are confined to the GHz frequency domain, limiting the class of systems they can interact with. Building upon the heavy fluxonium architecture introduced by ref. [2], we have developed a superconducting qubit with an unprecedentedly low transition frequency of 1.8 MHz [3]. Notably, we have demonstrated a qubit with a coherence time exceeding 30 μs, a sideband cooling scheme to prepare the qubit in a pure state with 97.7% fidelity, and single-shot readout capability. Moreover, by detecting a weak charge modulation by repeated qubit interrogation, we demonstrate the high-sensitivity of this qubit architecture to a nearly resonant AC-charge drive, proving its potential in a hybrid circuit scenario. We will finally present our recent efforts to achieve the strong coupling regime between this qubit and an ultra-coherent softly-clamped mechanical membrane. Ref.: [1] Y. Chu et al. Nature 563, 666 (2018). [2] H. Zhang et al. Physical Review X 11, 011010 (2021). [3] Najera et al. arXiv:2307.14329 (2023). In review at PRX.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : The strong coupling picture of twisted bilayer graphene provides a framework for understanding the correlated insulators observed in experiments. In this picture, the insulators are generalised quantum Hall ferromagnets which break certain symmetries. However, this picture is known to be incomplete and certain features are incompatible with experiments. In particular, recent scanning tunnelling have observed a state that breaks moiré translational symmetry. I will show that incorporating strain into a Hartree-Fock study of twisted bilayer graphene results in precisely the state observed in experiment. On the other hand, a state that respects moiré translational symmetry has been observed in low strain devices. Reconciling this observation with the model requires an additional ingredient, namely a specific phonon mode, the so-called K-phonon. I will investigate whether this phonon mode could be responsible for superconductivity.

Liens :Zurich University

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : Prof. Bardarson, invited CPTGA professor, will deliver a two session lecture touching on contemporary topics in disorder, topological and out-of equilibrium phenomena. This is the announcement of the second session, which will be split into two lectures, 45 min each, with a 15 min break in between. Coffee and pastries will be served during the break, and food and refreshments will be served to conclude the lecture series. The format is mainly directed to PhD students and postdocs, but senior researchers are of course welcome.

Liens :

• Séminaire LPMMC
• Personne à contacter : Bart van Tiggelen ()
• Lieu : G421

Résumé :

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Bosonic codes encoded using superconducting circuits offer a promising approach towards quantum error correction. Using continuous-variable systems, such as a harmonic oscillator realized in superconducting circuits, these codes compactly encode the quantum information in multi-photon states of a memory mode. Specifically, we use cat qubits for which the logical states 0 and 1 are two coherent states of a harmonic oscillator with opposite phases. These states are stabilized by leveraging dissipation to our advantage so that photon exchanges between the harmonic mode and its environment predominantly occur in pairs. In this way, "bit-flip" errors are exponentially suppressed as a function of the number of photons contained by the mode, at the modest cost of a linear increase in "phase-flip" errors. These errors could then be corrected by an additional layer of correction, such as a repetition code of cat qubits. At the heart of this thesis work is the introduction of a self-parametric superconducting circuit that non-linearly couples a mode containing the cat qubit to a dissipative mode whose frequency is set to twice that of the cat mode. Unlike previous implementations, this passive coupling does not require a parametric pump and achieves a high two-photon dissipation rate of around 2.16 MHz. Bit-flip errors are then avoided for a characteristic period of up to 0.3 s, with a moderate impact on phase-flip errors. In addition, we demonstrate universal control of this qubit using the two-photon dissipation to implement X, Y, and Z logic gates of arbitrary angles.

Liens :

• Soutenance de thèse
• Personne à contacter :
• Lieu : G421

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : Prof. Bardarson, invited CPTGA professor, will deliver a two session lecture touching on contemporary topics in disorder, topological and out-of equilibrium phenomena. This is the announcement of the first session, which will be split into two lectures, 45 min each, with a 15 min break in between. Coffee and pastries will be served during the break. The format is mainly directed to PhD students and postdocs, but senior researchers are of course welcome. Second lecture: Friday 17/11/23

Liens :

• Séminaire LPMMC
• Personne à contacter : Pierre Nataf ()
• Lieu : G421

Résumé : Extreme value theory is best known to predict disasters, for example in hydrology to anticipate floods or in epidemiology to quickly identify emerging diseases. Here, I would like to describe how it gives insight into some non-trivial effects in random spin chains. Indeed, despite a very good understanding of single-particle Anderson localization in one-dimensional quantum disordered systems, many-body effects are still full of surprises, a famous example being the interaction-driven many-body localization (MBL) [1, 2] in the random-field Heisenberg chain, whose non-interacting limit already shows non-trivial multiparticle physics, which allows to probe some general mechanisms using large-scale exact diagonalization. Here, I will focus in particular on a chain breaking mechanism occurring in the XX and the Heisenberg spin chains in a random field from an extreme value statistics perspective [3]. Supported by state-of-the-art numerical simulations at infinite temperature, this analysis leads to the striking observation of a sharp "extreme-statistics transition" in the Heisenberg chain as the disorder changes, which may coincide with the recently debated MBL transition.

References

1. F. Alet and N. Laflorencie, Many-body localization: An introduction and selected topics, Comptes Rendus Physique 19, 498 (2018).
2. D. A. Abanin, E. Altman, I. Bloch, and M. Serbyn, Many-body localization, thermalization, and entanglement, Rev. Mod. Phys. 91, 021001 (2019).
3. J. Colbois and N. Laflorencie, Breaking the chains: extreme value statistics and localization in random spin chains, arXiv:2305.10574 (2023).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Variational quantum algorithms and quantum machine learning are considered to be promising candidates for achieving a quantum advantage on noisy intermediate-scale quantum (NISQ) computers, which are limited by decoherence and gate errors. In such a setting, the quality of the result of an algorithm strongly depends on the fidelity and duration of the applied quantum logic gates, but also on the so-called circuit depth, i.e., the number of subsequent gates required to execute the algorithm. A versatile hardware-native gate set, including, e.g., a continuous set of two-qubit gates, can thus improve the performance of noisy intermediate-scale quantum computing by reducing the circuit depth [1]. After revisiting some fundamentals of superconducting transmon qubits, this talk presents a hardware-efficient implementation of a continuous set of controlled-phase gates, parameterized by the conditional phase. The approach is an extension of the controlled-phase gate from [2] and is based on the resonant interaction between two flux-tunable transmons. In this implementation, an arbitrary conditional phase can be achieved by tuning a single pulse parameter, and the vanishing time integral of the employed net-zero control pulses [2] provides robustness against memory effects stemming from long-time distortions in flux control lines. Furthermore, by activating the gate via flux control of both qubits, we demonstrate that the gate can be performed between far-detuned qubits, strongly suppressing residual interactions when the gate is off. We characterize the continuous gate set with cross-entropy benchmarking for fixed values of the conditional phase and for phases randomly drawn from a uniform distribution, confirming a consistently high gate fidelity over the full range of conditional phases. Ref.: [1] Lacroix et al., PRX Quantum 2020. [2] Negirneac et al., PRL 2021.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : What are the thermodynamic laws governing the energy flows and fluctuations of light interacting with quantum systems? Despite the widespread use of light in science, the thermodynamics of the fluctuations of photons at the nanoscale is still ill understood. A powerful framework to study energy fluctuations in small, complex, and far from equilibrium systems has emerged in the past two decades, stochastic thermodynamics (1), which was successfully used for electronic, colloidal and biological systems. One of the main achievements of stochastic thermodynamics is the derivation of fluctuation theorems, expressed as a symmetry relating entropy or energy fluctuations generated during a given forward process with the fluctuations of the time reversed process. Fluctuation theorems imply that, remarkably, regardless of the complexity of the system, the dynamics are constrained to obey a universal symmetry. The even more recent field of quantum stochastic thermodynamics aims at extending these ideas to quantum systems, in order to account for coherent effects and entanglement, but little has been done for light-matter interactions. In this talk, I will present our recent results, in the direction of building a framework for the thermodynamics of light-matter interactions at the fluctuating level. Central to the theory of stochastic thermodynamics is the notion of thermodynamic consistency: every thermodynamic quantity obtained from the stochastic dynamics should have a thermodynamic interpretation which is justified at the microscopic scale. On the other hand, a widespread approach to open quantum systems is to derive quantum master equations describing the reduced dynamics of a quantum system by tracing out its environment. A thermodynamically consistent quantum master equation should therefore preserve the symmetries of the fluctuation theorems. The first main result (2) of the talk is the identification of a new generalized quantum detailed balance condition which quantum master equations must satisfy in order to be thermodynamically consistent. We then focus of the case of coherent energy exchanges between lasers and atoms. The second main result (3) is the derivation of a fluctuation theorem for the energy transferred from a laser to an atom, valid even in the strong light-matter coupling regime. We then examine the thermodynamic consistency of master equations describing the dynamics of an atom coherently driven by a laser – the optical Bloch master equation and the Floquet master equation. We find that the Floquet master equation is fully consistent, i.e., satisfies the first and second laws of thermodynamics at the fluctuating level, while the Bloch equation is only consistent at the average level. Ref.: (1) U. Seifert. “Stochastic thermodynamics, fluctuation theorems and molecular machines”. Reports on progress in physics, 75(12):126001 (2012) (2) A. Soret, V. Cavina, M. Esposito. “Thermodynamic consistency of quantum master equations”, Phys. Rev. A, 106:062209 (2022) (3) A. Soret, M. Esposito. “Thermodynamics of coherent energy exchanges in quantum optics”. (draft in preparation).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : In a tunnel junction, electrons cross randomly the insulating barrier in very short times. This generates a broadband electromagnetic field. Quantum mechanics adds an ingredient to this: the voltage bias V imposed on the electrons translates into a correlation time h/eV in the current fluctuations and the generated field. With a time-resolved experiment we show how these correlations occur within a single cycle of an ac excitation at 4 GHz. This allows to generate squeezing and entanglement between far apart frequencies in the 10GHz range. In a broadband signal however, frequency is not a very relevant concept. We have developed an experiment to analyze the statistics of bicolor or broadband photons. From this we show that a tunnel junction excited at 12 GHz generates squeezed vacuum over the 3-9 GHz bandwidth. Our setup could be used to analyze any signal in the 1-10GHz range.

Liens :

• Séminaire LPMMC
• Personne à contacter : Léonie Canet ()
• Lieu : G421

Résumé : Soft matter systems are often governed by processes on multiple different time and length scales. Bridging the gap between these scales and numerically study emergent behavior of many-body systems thus usually requires the usage of coarse-grained models. In general situations in which the separation of time scales is incomplete, such coarse-grained models will feature non-Markovian memory effects and colored noise, as suggested by the Mori-Zwanzig formalism. In equilibrium systems, a plethora of methodologies have been derived to construct non-Markovian coarse-grained models. Their applicability, however, becomes questionable in non-equilibrium systems. In this talk I will give an introduction to non-Markovian modeling and provide examples for successful applications. Subsequently, I will explain why the presented methodology cannot be directly applied to non-equilibrium systems. At the end, I will present my personal roadmap towards developing theoretical foundations and data-driven coarse-grained models for out-of-equilibrium soft matter systems.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : [Beware of the unusual day/time: Monday 11am !]. The recent discovery of intrinsic supercurrent diode effect (1), and its prompt observation in a rich variety of systems, has shown that nonreciprocal supercurrents naturally emerge when both space- and time-inversion symmetries are broken. I will report on both dc and ac manifestations of the Josephson diode effect in the non-linear inductance in planar Josephson junctions, based on a ballistic Al/InAs-heterostructure that is exposed to an in-plane magnetic field Bp (2). At low Bp a non-reciprocal term is found in the inductance that is linear in Bp. At higher Bp a sign reversal of the magnetochiral term is observed that can be traced back to a 0-pi-like transition in the current-phase relation (3). Different avenues for a theoretical interpretation are discussed. As pronounced date tunability of both the phi_0 shift and the diode efficiency in an asymmetric SQUID device demonstrates that Rashba spin-orbit interaction provides a substantial contribution to the Josephson diode effect (4). Ref.: [1] F. Ando et al., Nature 584, 373 (2020). [2] C. Baumgartner et al., Nature Nanotech. 17, 39 (2022). [3] C. Baumgartner et al., Nature Nanotech. (2023). [4] S. Reinhardt et al., arXiv.2308.01061

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

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 :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

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

• [1] D. S. Petrov, Quantum Mechanical Stabilization of a Collapsing Bose-Bose Mixture, Phys. Rev. Lett. 115, 155302 (2015).
• [2] C. R. Cabrera, et al., Quantum liquid droplets in a mixture of Bose-Einstein condensates, Science 359, 301 (2018).
• [3] D. Rakshit, et al., Self-bound Bose–Fermi liquids in lower dimensions, New J. Phys. 21, 073027 (2019); T. Karpiuk, et al., Bistability of Bose–Fermi mixtures, New J. Phys. 22, 103025 (2020).
• [4] D. S. Petrov, C. Salomon, and G. V. Shlyapnikov, Weakly bound dimers of fermionic atoms, Phys. Rev. Lett. 93, 090404 (2004).
• [5] J. Givois, A. Tononi, and D. S. Petrov, Self-binding of one-dimensional fermionic mixtures with zero-range interspecies attraction, arXiv:2207.04742 , accepted for publication on SciPost Physics.
• [6] A. Tononi, J. Givois, and D. S. Petrov, Binding of heavy fermions by a single light atom in one dimension, Phys. Rev. A 106, L011302 (2022).
• [7] J. Givois, A. Tononi and D. S. Petrov, in preparation (2023).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Lieu : Rémy Lemaire K223, Institut Néel

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.

If time permits, I would also like to discuss current-phase relation measurements (CPR) in WTe2, which has been predicted to be a higher-order topological insulator. We show that one has to be careful in the interpretation of seemingly 4pi-periodic multivalued CPR data, since also this signature can be generated by spurious, but large, inductances that notoriously appear in contact regions due to alloying effects.

This work was done in collaboration with many colleagues over the last six years. I am grateful to them:(alphabetic order of group members participating in these studies) A. Baumgartner, C. Ciaccia, M. Endres, G. Fülöp, R. Haller, D. Indolese, R. Jha, M. Jung, A. Kononov, P. Karnatak, P. Kumbhakar, F. Oppliger, M. Osterwalder, J. Ridderbos, C. Schönenberger, D. Sufra, L. Wang, D. Weiss, H. Zheng.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Liens :

• Séminaire théorie
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : G 421
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 99884767935
  

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 :

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
• Lieu : F418, Institut Néel

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Robert Whitney ()
• Lieu : G421

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

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G 421

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 :

• Séminaire LPMMC
• Personne à contacter : Pierre Nataf ()
• Lieu : G421

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

Liens :Olivier GauthéCMTC, EPFL

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Nevill Mott, D420, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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 :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

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 :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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)

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle René Pauthenet J229, bâtiment J, LNCMI

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 :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

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

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : E424

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 :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

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 :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

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

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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 :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G 421

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 :

• Séminaire interne LPMMC
• Personne à contacter : Bart van Tiggelen ()
• Lieu : G421

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

• [1] I. Brevik, Phys.Rep. 52, 133 (1979)
• [2] G. B. Walker, D. G. Lahoz, and G. Walker, Can. J. Phys. 53, 2577 (1975)
• [3] G. L. J. A. Rikken and B. A. Van Tiggelen, Phys. Rev. Lett. 107, 170401 (2011)
• [4] A. Feigel, Phys. Rev. Lett. 92, 020404 (2004)
• [5] B. A. Van Tiggelen, D. Kawka, and G. L. J. A. Rikken, Eur. Phys. J. D 66, 272 (2012)

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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 :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Nevill Mott D420

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

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

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

• [1] G. Catelani, R. J. Schoelkopf, M. H. Devoret, and L. I. Glazman, Phys. Rev. B 84, 064517 (2011).
• [2] G. Catelani et al., Phys. Rev. B 86, 184514 (2012).
• [3] Z. Leghtas et al., Science 347, 853 (2015).
• [4] A. Grimm et al., Nature 584, 205 (2020).
• [5] G. Catelani and D. M. Basko, SciPost Phys. 6, 013 (2019).

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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 :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Annulé

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

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

• [1] Kasprzak J., Richard M., Kundermann S. et al. Nature 433 (2006)
• [2] Altman E., Sieberer L.M., Chen L., et al. PRX 5 (2015).
• [3] Fontaine Q., Squizzato D., Baboux F. et al. Nature 608 (2022).
• [4] Lee T. D., Huang K., Yang C. N. Physical Review 106 (1957).
• [5] He L., Sieberer L. M., Diehl S PRL 118 (2017).

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

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.

I will talk about the conditions for the emergence of the preformed Cooper pairs in materials hosting flat bands. As a particular example, a semimetal with a pair of three-band crossing points at which a flat band intersects with a Dirac cone will be considered. It will be shown that the flat band promotes local Cooper pair formation so that the system may be modeled as an array of superconducting grains. The delocalized states give rise to the phase-coherent superconductivity at low temperatures. The transition temperature between the preformed Cooper pair state and the phase-coherent state will be discussed.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

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 :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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 :

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

Annulé
L'orateur ne peut pas venir à Grenoble à cause de la grève SNCF. Le séminaire est reporté au 27 avril.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

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

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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 :

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

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 :

• Séminaire interne LPMMC
• Personne à contacter : Benoît Vermersch ()
• Lieu : G 421

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

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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

• Séminaire théorie
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : G 421
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

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 :

• Séminaire interne LPMMC
• Personne à contacter : Denis Basko ()
• Lieu : G 421

Liens :LPMMC

• Séminaire LPMMC
• Personne à contacter : Cécile Repellin ()
• Visio-conférence Zoom hybride.
  En présentiel : G 421
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629
  

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 :

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

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 liquid”, [10.1103/PhysRevX.8.031062] [2] “From Ionic Surfactants to Nafion through Convolutional Neural Networks”, with L. Dumortier, [10.1021/acs.jpcb.0c06172] [3] “Beating the amorphous limit in thermal conductivity by superlattices design”, with H. Mizuno and J.-L. Barrat, [10.1038/srep14116]

Liens :Stefano Mossa

• Séminaire interne LPMMC
• Personne à contacter : Léonie Canet ()
• Lieu : G 421

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

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G 421

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 :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

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 :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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 :

• Séminaire LPMMC / Néel
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

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 :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

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

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Nevill Mott - D420 - 3rd Floor

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.

Liens :Colloquium Giuseppe Carleo

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

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).

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