Séminaires LPMMC 2019
Philippe Jacquod (HES-SO (Haute Ecole Spécialisée de Suisse Occidentale), Suisse) | Détails Fermer |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
Developing InSb quantum wells as a platform for topological superconductivity le mardi 05 novembre 2019 à 14:00 |
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thèse de Nicolas Victorin (LPMMC) | Détails Fermer |
Multi-component Gauge Dependent Quantum Gases le vendredi 18 octobre 2019 à 10:00 |
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Riccardo Rossi (Flatiron Insitute, Smons Foundation, New York) | Détails Fermer |
New Routes Up the Strongly-Correlated Mountain le jeudi 17 octobre 2019 à 11:00 |
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Gerbold Menard (SPEC, CEA, Université Paris Saclay) | Détails Fermer |
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)
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thèse de Davide Squizzato (LPMMC) | Détails Fermer |
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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phase slips in one-dimensional superconductors (LPMMC) | Détails Fermer |
Quantum phase slips in one-dimensional superconductors le mercredi 02 octobre 2019 à 14:00 |
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MISSING (Université de Rochester) | Détails Fermer |
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 |
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 |
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 |
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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Gernot Schaller (TU Berlin) | Détails Fermer |
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 |
Characterizing photon-photon interactions and correlated noise in nanophotonic systems le vendredi 13 septembre 2019 à 11:00 |
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Eric Woillez (Technion) | Détails Fermer |
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 |
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 |
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 |
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 |
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 |
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.
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David Rodriguez Fernandez | Détails Fermer |
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 |
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 |
Quantum coherence in the localization transition le mercredi 10 juillet 2019 à 11:00 |
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Annulé
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Pascal Simon (LPS - Orsay) | Détails Fermer |
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 |
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 |
(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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
Transition-metal pnictides: electrons and phonons le mardi 30 avril 2019 à 11:00 |
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Gabriel Polinario (Université fédérale de Rio, Brésil) | Détails Fermer |
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 |
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 |
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 |
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 |
(titre non communiqué) le mardi 09 avril 2019 à 14:00 |
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Julien Toulouse | Détails Fermer |
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 |
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 |
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.
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Xavier Montiel (University of London) | Détails Fermer |
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.
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Benjamin Lenz | Détails Fermer |
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 |
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 |
Modes collectifs de "Higgs" dans les condensats fermioniques le mercredi 13 mars 2019 à 11:00 |
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Hélène Sueur (CSNSM, Univ. Paris-Sud) | Détails Fermer |
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 |
Shaping the properties of condensed-matter systems with light le mercredi 06 mars 2019 à 11:00 |
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Boris Brun (UCLouvain, Belgium) | Détails Fermer |
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 |
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 |
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 |
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 |
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 |
Radiative corrections in planar Dirac liquids le vendredi 08 février 2019 à 11:15 |
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Annulé
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Antonin Coutant | Détails Fermer |
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 |
Quantum many-body physics with nonlinear propagating light le mercredi 06 février 2019 à 11:00 |
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CPTGA 1er février (Café (Peter Grünberg Institute) | Détails Fermer |
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 |
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 |
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
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Dmitry Bagrets (University of Cologne) | Détails Fermer |
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 |
Range-separated DFT for molecular excitation energies le mercredi 23 janvier 2019 à 13:30 |
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Giovanni Martone (LPTMS, Orsay) | Détails Fermer |
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 |
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 : |