Séminaires LPMMC 2021
Irénée Frérot (Institut Néel)  Détails Fermer 
Inferring Bell's inequalities from correlation functions in quantum manybody systems le vendredi 3 décembre 2021 à 11h00 

Résumé : The violation of Bell's inequalities is probably the most striking manifestation of quantum entanglement, excluding all possible descriptions of a set of data in terms of a classical model (a "locally causal" model, in the words of J. S. Bell). While nonlocality (that is: the violation of Bell's inequalities) has been demonstrated since the early 1980s for pairs of twolevel systems, its exploration in more complex ensembles, and especially in quantum manybody systems, has remained quite unexplored until very recently. This is partly due to the lack of sufficiently scalable and flexible theoretical tools to prove the impossibility of any classical description of a given set of correlations. Yet, we will show in this talk that it is possible to devise such tools, resorting to an exact mapping between Bell's locallycausal models, and classical Ising models (or generalizations thereof). This leads to a constructive approach to probe Bell's nonlocality in manybody systems, in which one tries to actively reproduce some set of observed correlations with a classical Ising model  a socalled inverse Ising problem, wellknown in many areas of data science. In contrast to typical datascience instances, however, it is the failure to find such a classical explanation to the data which ultimately leads to unveil its nonlocality, in the form of new Bell's inequalities inferred from the data themselves. Both the general framework for this approach will be presented, as well as some specific results and extensions. 
Adam Rançon (PhLAM, Lille)  Détails Fermer 
Effective thermalization of a manybody dynamically localized Bose gas le vendredi 26 novembre 2021 à 11h00 

Résumé : Dynamical localization is the quantum chaos analog of the Anderson localization of disordered systems, but in momentum space, as observed in the atomic quantum kicked rotor. Interactions tend to destroy (Anderson) localization, but strong enough disorder give rise to ManyBody Localization (MBL). Whether interactions destroy dynamical localization or not is addressed here. We discuss the effect of interactions on a onedimensional kicked Bose gas in the strong interaction limit (Tonks regime), and show that dynamical localization is not destroyed by the interactions, in the sense that the energy of the system stays finite at long times. We show that this steadystate is ergodic, i.e. described by a thermal density matrix, with an effective temperature that depends on the kicking parameters and the number of particles. The onebody reduced density matrix of the gas decays exponentially at large distance, implying absence of coherence, while the momentum distribution's tail at large momenta is characterized by an effectively thermal Tan contact. 
Olivier Coquand (Laboratoire Charles Coulomb, Montpellier)  Détails Fermer 
Rheology of granular liquids le mercredi 24 novembre 2021 à 11h00 

Résumé : Granular liquids are ubiquitous around us, from the physics of geological phenomena to the food processing industry. However, the description of their behaviour remains a challenge for fundamental physics. One of the most successful of such approaches is the socalled "µ(I)" law, a phenomenological law that describes with good accuracy experimental and numerical results, but still lacks theoretical support. In this seminar, I will present our recent works on the subject. From a set of fundamental equations, we managed to explain the µ(I) law from the competition between different time scales associated with fundamental processes within the granular flow. This shed a new light on the physics of these systems, and represents a first step towards the establishment of a complete theoretical framework to describe the physics of dense granular flows. 
Mitali Banerjee (EPFL)  Détails Fermer 
Identifying nonabelian anyons in quantum Hall states le mardi 23 novembre 2021 à 14h00 

Résumé : Quantum Hall effects in 2D electron gas exemplify the earliest class of topological phases in solid state physics, characterized by an insulating bulk and gapless edge excitations. The quantum Hall effect has been a source of many concepts that have become essential in more general quantum manybody problems. The fractional quantum Hall states host fractional charges, neutral excitations, and unlike the integer quantum Hall states, they generally support counterpropagating chiral edge modes. The theoretical and the experimental search for such fractionalized particles continues to attract extensive attention. One reason for the enthusiasm is fundamental  as fractionalization can brilliantly exemplify the rich emergent longrange behavior that manybody systems can exhibit. Another reason is more pragmatic, as certain nonabelian fractionalized excitations form the basis of topological quantum computers that promise inherent immunity against errors. In this talk, I will present the role of neutral modes in the quantum Hall effect, as the highly sought after nonabelian excitations are often “chargeneutral.” Yet, there are bosonic neutral modes that plague the singleparticle interferometers in the fractional quantum Hall regime. 
Anastasia Gorbunova  Détails Fermer 
(titre non communiqué) le vendredi 19 novembre 2021 à 14h00 


Olesia Dmytruk (Collège de France)  Détails Fermer 
Theory of highpower excitation spectra of rfSQUID le mercredi 17 novembre 2021 à 10h00 

Résumé : Electrical devices based on Josephson junctions have been at the center of research attention for many years. Such setups are very versatile, as they can be used as qubits or (topological) Josephson bifurcation amplifiers. Moreover, Josephson junctions with external timedependent driving are suitable platforms for studying nonlinear phenomena. Absorption spectroscopy of Josephson junction is a powerful experimental technique that can be used to study mesoscopic systems in a wide frequency range. Very recently a novel Josephson junction spectrometer with broad bandwidth and variable coupling strength was implemented and used to perform highpower spectroscopy on an rfSQUID [1]. In this talk, I will discuss the theory of linear and nonlinear spectroscopy of an rfSQUID coupled to a Josephson spectrometer [2]. Experimental measurements on this system have shown a strongly nonlinear absorption lineshape, whose current peak maximum undergoes a forwardbackward bending transition depending on the value of the rfSQUID phase [1]. I will show that this transition can be qualitatively understood by mapping the dynamics of the driven rfSQUID onto a generalized Duffing oscillator, with tunable drive and nonlinearity, undergoing a bifurcation. Finally, I will demonstrate that in order to quantitatively reproduce the experimental data reported in [1], it is crucial to include the feedback from the loadline, leading to an additional source of nonlinearity. References[1] J. Griesmar, R. H. Rodriguez, V. Benzoni, J.D. Pillet, J.L. Smirr, F. Lafont, and Ç. Ö. Girit, arXiv:2106.02632.[2] O. Dmytruk, R. H. Rodriguez, Ç. Ö. Girit, and M. Schiró, arXiv:2107.07971. 
Andrew Higginbotham (IST Austria)  Détails Fermer 
Probing quantum materials with circuit quantum electrodynamics le mardi 16 novembre 2021 à 14h00 

Résumé : I will discuss two initial experiments in my group’s effort to probe quantum materials with circuit quantum electrodynamics. In the first experiment, we use coplanar waveguide resonators to probe superconductivity in a twodimensional AlInAs hybrid system. Our observations qualitatively agree with a theory of induced p±ip pairing that evolves into BogoliubovFermi arcs at high magnetic field. In the second experiment, we study arrays of highimpedance Josephson junctions as a model system for the superconductorinsulator phase transition. Theory predicts that such chains should be insulating, but in experiment superconducting behavior is often observed. Fixing all parameters from microwave measurements, we find that apparent superconducting behavior can be quantitatively understood as the hightemperature fate of a melted insulator. 
Ad Lagendijk (Twente)  Détails Fermer 
Optimizing Optical Wavefronts le lundi 15 novembre 2021 à 11h00 

Résumé : 
Cyril Élouard (INRIA Grenoble et ENSLyon)  Détails Fermer 
Understanding the efficiency of a nanoelectronic refrigerator fueled by a continuous quantum measurement le vendredi 12 novembre 2021 à 11h00 

Résumé : Recently, the booming field of quantum thermodynamics has been exploring the properties of heat engines involving phenomena specific to the quantum world, such as entanglement, coherent superpositions or quantum measurement. In particular, it has been noticed that the unavoidable perturbation induced by a quantum measurement can be seen as a thermodynamic transformation, generating a change of entropy and energy. A measuring apparatus can be seen as analogous to a heat bath, and engines able to convert the energy provided by the measurement into heat have been proposed. However, the performances of these engines cannot be completely understood without a deeper description of the energy flows involved in a quantum measurement. We perform such analysis in the case of a refrigerator fueled by a continuous measurement of charge in a nanoelectronic system. We show that for a well chosen range of parameters, the measurementinduced perturbation induces a heat flow from a cold to a hot electron reservoirs. By analyzing a realistic microscopic model for the measuring apparatus – based on an electronic tunnel junction – we compute the work and heat flows involved by the measurement and evaluate the thermodynamic efficiency of the refrigerator as a function of the parameters of the measuring apparatus. We show how the engine complies with Carnot efficiency, paving the road towards energetic optimization of quantum measurements and protocols that involve them. 
Chuan Li (Twente University (NL))  Détails Fermer 
Dirac semimetalbased quantum devices towards 1D topological superconductivity le mardi 9 novembre 2021 à 14h00 

Résumé : One of the main directions to realize the topological quantum bit is combining topological materials with conventional superconductors. The notion of topological phases has been extended to higherorder and has been generalized to different dimensions. In the last few years, our research demonstrates the possibility of realizing the topological superconductivity in engineered 3D Dirac semimetals [1,2] and their 1D hinge states. Particularly, Cd3As2 is predicted to be a higherorder topological semimetal, possessing threedimensional bulk Dirac fermions, twodimensional Fermi arcs [3], and onedimensional hinge states [4]. These topological states have different characteristic length scales in electronic transport, allowing one to distinguish their properties when changing sample size. [1] Li, C. et al. Nat. Mater. 17, 875–880 (2018). [2] Wang, A. Q. et al. Phys. Rev. Lett. 121, 237701 (2018). [3] Li, C.Z. et al. Nat. Commun. 11, 1150 (2020). [4] Li, C.Z. et al. Phys. Rev. Lett. 124, 156601 (2020). Liens :[Twente University (NL)] 
Johannes Motruk  Détails Fermer 
Fourspin terms and the origin of the chiral spin liquid in Mott insulators on the triangular lattice le vendredi 5 novembre 2021 à 11h00 

Résumé : At strong repulsion, the triangularlattice Hubbard model is described by S=1/2 spins with nearestneighbor antiferromagnetic Heisenberg interactions and exhibits conventional 120° order. When decreasing the interaction, additional fourspin interactions are naturally generated from the underlying Mottinsulator physics of electrons. Although these interactions have historically been connected with a gapless ground state with emergent spinon Fermi surface (SFS), we find that at physically relevant parameters, they stabilize a chiral spinliquid (CSL) of KalmeyerLaughlin (KL) type, clarifying observations in recent studies of the Hubbard model. We then present a selfconsistent solution based on a meanfield rewriting of the interaction to obtain a Hamiltonian with similarities to the parent Hamiltonian of the KL state, providing a physical understanding for the origin of the CSL. Using a combination of the infinite density matrix renormalization group and exact diagonalization, we also study the wider phase diagram of the spin model, shedding light on the fate of the SFS state. Finally, we will comment on experimental systems in which triangular lattice Hubbard model physics might be observed. Ref.: Phys. Rev. Lett. 127, 087201 (2021). 
Yuli Nazarov (TU Delft)  Détails Fermer 
Semiclassical topology in multiterminal superconducting nanostructures: protectionunprotection transition, and the role of interaction le mardi 26 octobre 2021 à 14h00 

Résumé : We review the conceprt of semiclassical topology in multiterminal superconducting nanostructures where the gapped states are characterzed by topological numbers. The topologicical protection requires these states to be separated by the gapless one: sometimes this work, sometimes not so a protectionunprotection transition can happen. We build up a general theoretical description of the transition vicinity in the spirit of Landau theory. We also discuss the interaction effects near the special points of the spectrum and prove a drastic effect of weak interaction. 
Pedram Roushan (Google)  Détails Fermer 
Time Crystals, the quest for a new phase of matter le mardi 19 octobre 2021 à 16h00 

Résumé : Quantum manybody systems display rich phase structure in their lowtemperature equilibrium states. However, much of nature is not in thermal equilibrium. Remarkably, it was recently predicted that outofequilibrium systems can exhibit novel dynamical phases that may otherwise be forbidden by equilibrium thermodynamics, a paradigmatic example being the discrete time crystal (DTC). Concretely, dynamical phases can be defined in periodically driven manybody localized systems via the concept of eigenstate order. In eigenstateordered phases, the entire manybody spectrum exhibits quantum correlations and longrange order, with characteristic signatures in latetime dynamics from all initial states. It is, however, challenging to experimentally distinguish such stable phases from transient phenomena, wherein few select states can mask typical behavior. Here [1] we implement a continuous family of tunable CPHASE gates on an array of superconducting qubits to experimentally observe an eigenstateordered DTC. We demonstrate the characteristic spatiotemporal response of a DTC for generic initial states. Our work employs a timereversal protocol that discriminates external decoherence from intrinsic thermalization, and leverages quantum typicality to circumvent the exponential cost of densely sampling the eigenspectrum. In addition, we locate the phase transition out of the DTC with an experimental finitesize analysis. Our results establish a scalable approach to study nonequilibrium phases of matter on current quantum processors. 1. Mi et al, arXiv:2107.13571 
Shahal Ilani (Weizmann Institute)  Détails Fermer 
What Is the Ultimate Conductance of Hydrodynamic Electrons? le mardi 12 octobre 2021 à 14h00 

Résumé : Electrical resistance usually originates from lattice imperfections. However, even a perfect lattice has a fundamental resistance limit, given by the Landauer conductance of its discrete modes. This resistance, shown by Sharvin to appear at the contacts to electronic devices, sets the ultimate conduction limit of noninteracting electrons. Recent years have seen growing evidence of hydrodynamic electronic phenomena, prompting recent theories to ask whether liquid electrons can radically break this fundamental LandauerSharvin limit. Here, we use singleelectrontransistor imaging of electronic flows in highmobility graphene Corbino devices to answer this question. First, by imaging ballistic flows at low temperatures, we observe a LandauerSharvin resistance that does not appear at contacts but is instead distributed throughout the bulk. This underpins the phasespace origin of this resistance  as emerging from spatial gradients in the number of conduction modes. At elevated temperatures, we identify and account for the contribution of electronphonon scattering and reveal the pure hydrodynamic flow. Strikingly, we find that hydrodynamic electron flow eliminates completely the bulk LandauerSharvin resistance. Finally, by adding small magnetic fields, we image swirling magnetohydrodynamic flows, revealing the key emergent lengthscale predicted by hydrodynamic theories – the Gurzhi length. These observations demonstrate that electronic fluids can dramatically overcome the limitations of ballistic electrons, with important implications for fundamental science and future technologies. 
Olivier Cepas (Institut Néel)  Détails Fermer 
Bilan carbone de l'Institut Néel le vendredi 8 octobre 2021 à 11h00 

Résumé : Après un résumé sur les émissions de gaz à effet de serre et les objectifs de réduction annoncés, je vous présenterai les différentes rubriques du bilan carbone de l'Institut Néel (réalisé par le collectif "labo en transition"): missions, achats, déplacements domiciletravail, énergies, expériences sur les grands instruments, informatique etc. J'exposerai brièvement les politiques de réduction envisagées à court terme au laboratoire. Liens :[Institut Néel] 
Zoltan Scherubl (CEA Grenoble / Budapest University)  Détails Fermer 
From Cooper pair splitting to the nonlocal spectroscopy of a Shiba state le mardi 5 octobre 2021 à 14h00 

Résumé : Recent years’ hunt for the Majorana fermions generated a genuine interest in superconducting subgap (YuShibaRusinov or shortly Shiba) states both in the field of STM measurements and hybrid semiconductorsuperconductor devices. The better understanding of the trivial Shiba states paves the way to engineering them into topological states. Previously, devices, quite similar to the hybrid Majorana devices, were investigated from the viewpoint of Cooper pair splitting. These works aimed to generate spatially separated entangled electron pairs in a controlled way. In my talk, after shortly introducing these research fields, I will present a recent work, where we studied the interplay of a Shiba state and the splitting process in a Cooper pair splitter device. Changing the tunnel coupling to one of the normal electrodes allows to tune the device from limit of Cooper pair splitting to the nonlocal spectroscopy of the Shiba state. First, focusing on the latter limit, I will demonstrate that we could observe the Shiba state at much larger distances than in recent STM measurements. Then, I will discuss how the presence of the Shiba state, producing a nonlocal signal similar to the pair splitting, affects splitting efficiency. Z.S. et al., Nature Communications 11, 1834 (2020) Z.S. et al., ArXiv: 2108.12155 
Théotime Girardot (LPMMC)  Détails Fermer 
Meanfield approximation for the anyon gas le mercredi 29 septembre 2021 à 14h00 

Résumé : This thesis is dedicated to the study of ground states of an anyon gas in the large number of particle limit. In twodimensional space there are possibilities for quantum statistics continuously interpolating between the bosonic and the fermionic one. Quasiparticles obeying such intermediate statistics are called anyons. They can be described as ordinary bosons and fermions with interactions through longrange magnetic potentials, generated by magnetic charges carried by each particle. We study the almostbosonic and almostfermionic limit. We obtain rigorous results for the convergence of their ground state energies and the associated minimizers to those of effective models. The ground state of almostbosonic anyons converges to the infimum of a Hartreelike functionnal and its minimizers to a convex superposition of factorized pure states. The particles behave like independent, identically distributed bosons interacting via a selfconsistent magnetic field. The ground state energy of almostfermionic anyons converges to the infimum of a ThomasFermi energy and its minimizers to measures associated with the corresponding semiclassical problem. More precisely, the ground state of our Hamiltonian converges to that of a classical modified Vlasov energy whose minimization leads to the ThomasFermi functional. The Vlasov energy is endowed with a selfconsistent magnetic field, a landmark of anyonic statistics. The ground state of the Vlasov energy displays anyonic behavior in its momentum distribution. 
Louk Rademaker (Department of Theoretical Physics, Université de Genève)  Détails Fermer 
Symmetry breaking and Chern insulators in twisted graphene structures le mardi 28 septembre 2021 à 14h00 

Résumé : Twisted bilayer graphene (tBG) and variants like twisted monolayerbilayer graphene (tMBG) were proposed to be a platform for strongly correlated physics akin to the cuprate family. However, I will show that many of the observed interacting phenomena can be explained in terms of breaking of spin/valley symmetry. This can lead to a quantum anomalous Hall effect in the absence of a field, as I will show for tMBG [1]. In large magnetic fields the same spin valley symmetry breaking leads to a series of Chern insulator states [2]. Finally, I will briefly discuss the possibility of genuine strong correlated physics in Moiré structures. [1] Rademaker, Protopopov, Abanin, Phys. Rev. Research, 2, 033150 (2020). [2] Saito, Ge, Rademaker, Watanabe, Taniguchi, Abanin, Young, Nature Physics, 108, 12233 (2021). Liens :[Louk Rademaker] 
Victor Bittencourt (MaxPlanck Institute for the Physics of light)  Détails Fermer 
Magnonphoton coupling in dispersive media le mercredi 22 septembre 2021 à 11h00 

Résumé : The quantized magnetic excitations (magnons) of magnetic dielectrics can couple to microwave and optical photons, making such systems prospect hybrid quantum platforms. Prospect applications of the system includes quantum transduction between optics and microwaves, and generation of nonclassical macroscopic magnetization states. Nevertheless, one of the limitations of the system is the weakness of the coupling between magnons and optical photons  a consequence of the weakness of the magnetooptical effects in dielectrics. In this talk, I present the concept of a novel platform for magnonphoton interaction based on a magnetic epsilonnearzero (ENZ) medium, in which strong coupling can be achieved. In ENZ media, the diagonal components of the permittivity tensor vanish at specific frequencies, yielding an enhancement of nonlinear effects and secondary responses, such as magnetooptical effects. After a brief review of magnonphoton interactions in magnetic dielectrics and the stateoftheart optomagnonic systems, I will present a quantisation scheme that phenomenologically includes dispersion, which is the responsible for the ENZ behaviour. Within this framework, the enhancement of magnetooptical effects in ENZ media implies an enhancement of the magnonphoton coupling, which can surpass both magnon and photon decay rates, thus reaching the strong coupling regime. Finally, I discuss a possible measurement scheme for the strong magnonphoton coupling via the optics power spectrum, which exhibits multiple sidebands characteristic of the strong parametric coupling. The proposal of an ENZbased magnonphoton platform pushes forward novel designs for magnonic systems, and can be the starting point for developing new magnonbased protocols. Liens :[Victor Bittencourt][MaxPlanck Institute for the Physics of light] 
Matias Urdampilleta (Institut Néel)  Détails Fermer 
Spin Readout in CMOS Devices le mardi 21 septembre 2021 à 14h00 

Résumé : Over the last fifty years, the CMOS (ComplementaryMetalOxideSemiconductor) electronics industry has been continuously scaling down transistors in size, to increase performance and reduce power consumption. Nowadays, the smallest transistors in industry achieve 5nm features. As a result, those silicon structures tend to exhibit undesirable quantum effects for a classical transistor which appear to be new research opportunities for quantum information processing. In particular, it is nowadays possible to trap single electron spins in silicon quantum dots and perform high fidelity quantum gates (i). These demonstrations combined with the intrinsic properties of the silicon lattice (low spin orbit and hyperfine interaction) make CMOS device an excellent candidate for scalable quantum architectures. This presentation, will focus on the probing of electronic spins trapped in a CMOS device. In the first part I will present the basic characterization of a single electron spin and valley physics detected by standard charge sensing (ii). In the second part, I will show how we can operate a small array of quantum dot (iii) and how we can measure spin states inside. Finally, I’ll present the measurement of higher spin states by probing the quantum capacitance of the system (iv) and how we intend to scale up this measurement technique. (i) Veldhorst, M. et al. Nat. Nanotechnol. 9, 981 (2014). (ii) Spence, C. et al. In preparation (2021). (iii) Chanrion, E. et al. Phys. Rev. Appl. (2020). (iv) Lundberg, T. et al. Phys. Rev. X (2020). 
Philippe Campagne Ibarcq (INRIA/ENS Paris)  Détails Fermer 
Quantum error correction of a qubit encoded in grid states of an oscillator le mardi 14 septembre 2021 à 14h00 

Résumé : In 2001, Gottesman, Kitaev and Preskill (GKP) proposed to encode a fully correctable logical qubit in grid states of a single harmonic oscillator. Although this code was originally designed to correct against shift errors, GKP qubits are robust against virtually all realistic error channels. Since this proposal, other bosonic codes have been extensively investigated, but only recently were the exotic GKP states experimentally synthesized and stabilized. These experiments relied on stroboscopic interactions between a target oscillator and an ancillary twolevel system to perform nondestructive error syndrome measurements for the GKP code. In this talk, I will review the fascinating properties of the GKP code and the conceptual and experimental tools developed for trapped ions and superconducting circuits, which enabled quantum error correction of a logical GKP qubit encoded in a microwave cavity. I will describe ongoing efforts to suppress further logical errors, and in particular to avoid the apparition of uncorrectable errors stemming from the noisy ancilla involved in the GKP error syndromes detection. 
Tobias Goecke (Deutscher Wetterdienst (DWD))  Détails Fermer 
Towards scaleconsistent cloud models using methods from statistical physics le mercredi 8 septembre 2021 à 11h00 

Résumé : Methods from statistical physics have been applied to describe characteristics of cloud fields. Analogies to twodimensional lattice models have been found, in particular for tropical convection. Some physical parametrisations of clouds make use of those ideas. But so far these parametrisations are applied in single model columns, such that horizontal correlations are mostly neglected. The analogy of cloud fields to percolation models might lend itself to describe those horizontal correlations as well as cloud size distributions. To apply those descriptions on the various scales on which physical parametrisations within weather and climate modelling are used, the models should be defined in a scaleaware manner. Here, the functional renormalization group might be a method to derive an effective model on a given scale, in order to arrive at a most consistent treatment of physical processes across models of different resolution. 
Maxime et Robin (LPMMC)  Détails Fermer 
(titre non communiqué) le lundi 19 juillet 2021 à 10h30 

Liens :[LPMMC] 
Nathan et Valentin (LPMMC)  Détails Fermer 
(titre non communiqué) le lundi 5 juillet 2021 à 10h30 

Liens :[LPMMC] 
Florent Lecocq (NIST)  Détails Fermer 
Hardware for efficient measurements and massive signal delivery in superconducting quantum processors le mardi 29 juin 2021 à 14h00 

Résumé : There are many challenges to scaling the size of superconducting quantum computers. First and foremost, building processors with increasing numbers of qubits and longer coherence remains a daunting task. However, the success of superconducting quantum computing will also hinge on the development of many supporting technologies such as cryogenics, signal delivery, and microwave readout. Here I will discuss approaches to two of these bottlenecks, lying at the interface between quantum physics and engineering. First, I will discuss the challenges of wiring a millionqubit processor with coaxial lines, and how using photonic links can enable the use of optical fibers instead [1]. Second, I will discuss why superconducting quantum processors need nonreciprocal components, what are the limitations of the conventional microwave circulators, and how we intend to replace them with integrable nonreciprocal devices based on multimode parametric interactions [2]. [1] F. Lecocq, et al, Nature 591, 575579 (2021) [2] F. Lecocq, et al, Phys. Rev. Lett. 126, 020502 (2021) Liens :[NIST] 
Salvatore Francesco Emanuele Oliviero (University of Massachussetts)  Détails Fermer 
Quantum Chaos is Quantum le lundi 28 juin 2021 à 14h30 

Résumé : It is well known that a quantum circuit on nqubits composed of Clifford gates with the addition of k nonClifford gates can be simulated on a classical computer by an algorithm scaling polynomially in n and an exponentially in k. Our work in this direction focused on finding the number of resources needed to simulate a quantum chaotic behavior. In this talk, I will review some notions of simulability of quantum circuits and notions of Entanglement. I will show that the doping of a Clifford circuit with O(n) single qubit non Clifford resources is both necessary and sufficient to drive the transition to universal fluctuations of the purity. 
Jonathan Atteia (Paris Sud)  Détails Fermer 
Skyrmions and magnons in the graphene quantum Hall ferromagnet le vendredi 25 juin 2021 à 11h00 

Résumé : As a consequence of the approximate spinvalley symmetry in graphene, the ground state of electrons in graphene at charge neutrality is a particular SU(4) quantum Hall ferromagnet. If only the Coulomb interaction is taken into account, this ferromagnet can appeal either to the spin degree of freedom or equivalently to the valley pseudospin degree of freedom. This freedom in choice is then limited by subleading energy scales that explicitly break the SU(4) symmetry, the simplest of which is given by the Zeeman effect that orients the spin in the direction of the magnetic field. In addition, there are also valley symmetry breaking terms that can arise from shortrange interactions or electronphonon couplings. Here, we build upon the phase diagram in order to identify the different skyrmions at filling $\nu=0$ and spin waves at $\nu=\pm1$ that are compatible with these types of quantumHall ferromagnets. Similarly to the ferromagnets, the skyrmions at charge neutrality are described by the $\text{Gr}(2,4)$ Grassmannian at the center, which allows us to construct the skyrmion spinors. The different skyrmion types are then obtained by minimizing their energy within a variational approach, with respect to the remaining free parameters that are not fixed by the requirement that the skyrmion at large distances from their center must be compatible with the ferromagnetic background. We show that the different skyrmion types have a clear signature in the local, sublatticeresolved, spin magnetization, which is in principle accessible in scanningtunneling microscopy and spectroscopy. In addition to the skyrmions, we also describe the generalized spin waves at $\nu=\pm1$, where one encounters pure spin waves, valleypseudospin waves as well as more exotic entanglement waves that have a mixed spinvalley character. Most saliently, the SU(4) symmetrybreaking terms do not only yield gaps in the spectra, but under certain circumstances, namely in the case of residual groundstate symmetries, render the originally quadratic (in the wave vector) spinwave dispersion linear. 
Daniel Vert (CEA List)  Détails Fermer 
Benchmarking quantum annealing machines for solving combinatorial problems le mardi 22 juin 2021 à 14h00 

Résumé : Quantum computing is receiving renewed interest with recent announcements from several players. The most obvious reason is that some quantum algorithms can solve in polynomial time the same tasks that are currently considered nonpolynomial on classical computers. In recent years, analog quantum machines have appeared, of which the computers currently marketed by the Canadian company DWave are the first representatives, operating on a quantum accelerated annealing principle. From an abstract point of view, such a machine can be considered as an oracle specialized in solving an NPhard optimization problem with an algorithm functionally analogous to simulated annealing but with quantum acceleration. In addition to the formal analogies between simulated annealing and quantum annealing, there is an analogy between the current state of the art and that of simulated annealing when it was introduced. The work carried out in the framework consists in understanding the functioning of such a machine and in determining to what extent it contributes to the solution of combinatorial problems. The challenge is to know whether or not there is an acceleration of a quantum nature in these machines compared to other computers. The idea is to provide a first study on the behavior of quantum annealing when confronted with a problem known to trap classical annealing. The bipartite matching problem was specifically chosen to be difficult to solve using simulated annealing. Comparing the performances between these two annealers gives a first benchmark and allows to better characterize their potentials. 
Francesco Vercesi (LPMMC)  Détails Fermer 
Phase transitions in 1D excitonpolariton le lundi 21 juin 2021 à 10h30 

Liens :[LPMMC] 
Johannes Majer (University of Science and Technology of China)  Détails Fermer 
Hybrid Quantum Systems: Coupling Diamond Color Centers to Superconducting Cavities le mardi 15 juin 2021 à 14h00 

Résumé : Hybrid quantum systems based on spinensembles coupled to superconducting microwave cavities are promising candidates for robust experiments in cavity quantum electrodynamics (QED) and for future technologies employing quantum mechanical effects. In particular the electron spins hosted by nitrogenvacancy centers in diamond. We used this system to study a broad variety of effects, such as cavity protection effect and hole burning which can extend the coherence time and reduce dephasing. Furthermore, this platform allows to study superradiance (1) and the coupling of spins over macroscopic distances. We use a dispersive detection scheme based on cQED to observe the spin relaxation of the negatively charged nitrogen vacancy center in diamond. We observe exceptionally long longitudinal relaxation times T1 of up to 8h (2). To understand the fundamental mechanism of spinphonon coupling in this system we develop a theoretical model and calculate the relaxation time abinitio. The calculations confirm that the low phononic density of states at the NV− transition frequency enables the spin polarization to survive over macroscopic timescales. (1) Andreas Angerer, Kirill Streltsov, Thomas Astner, Stefan Putz, Hitoshi Sumiya, Shinobu Onoda, Junichi Isoya, William J. Munro, Kae Nemoto, Jörg Schmiedmayer, and Johannes Majer, Superradiant emission from colour centres in diamond, Nature Physics 14, 11681172 (2018) (2) T. Astner, J. Gugler, A. Angerer, S. Wald, S. Putz, N. J. Mauser, M. Trupke, H. Sumiya, S. Onoda, J. Isoya, J. Schmiedmayer, P. Mohn, and J. Majer, Solidstate electron spin lifetime limited by phononic vacuum modes, Nature Materials 17, 313317 (2018) 
Silvia Viola Kusminskiy (Max Planck Institute for the science of light)  Détails Fermer 
Quantum magnonics: Quantum optics with magnons le mardi 8 juin 2021 à 14h00 

Résumé : In the last five years, a new field has emerged at the intersection between Condensed Matter and Quantum Optics, denominated “Quantum Magnonics”. This field strives to control the elementary excitations of magnetic materials, denominated magnons, to the level of the single quanta, and to interface them coherently to other elementary excitations such as photons or phonons. The recent developments in this field, with proof of concept experiments such as a singlemagnon detector, have opened the door for hybrid quantum systems based on magnetic materials. This can allow us to explore magnetism in new ways and regimes, has the potential of unraveling quantum phenomena at unprecedented scales, and could lead to breakthroughs for quantum technologies. A predominant role in these developments is played by cavity magnonic systems, where an electromagnetic cavity, either in the optical or microwave regime, is used to enhance and control the interaction between photons and magnons. In this talk, I will introduce the field and present some theoretical results from our group which aim to push the boundaries of the current state of the art. Liens :[Silvia Viola Kusminskiy] 
Alessio Chiocchetta (Köln Universität)  Détails Fermer 
Emergent KardarParisiZhang phase in quadratically driven condensates le lundi 7 juin 2021 à 10h30 

Résumé : Abstract: In bosonic gases at thermal equilibrium, an external quadratic drive can induce a BoseEinstein condensation described by the Ising transition, as a consequence of the explicitly broken U(1) phase rotation symmetry down to Z2. However, in physical realizations such as excitonpolaritons and nonlinear photonic lattices, thermal equilibrium is lost and the state is rather determined by a balance between losses and external drive. A fundamental question is then how nonequilibrium fluctuations affect this transition. Here, we show that in a twodimensional drivendissipative Bose system the Ising phase is suppressed and replaced by a nonequilibrium phase featuring Kardar ParisiZhang (KPZ) physics. Its emergence is rooted in a U(1)symmetry restoration mechanism enabled by the strong fluctuations in reduced dimensionality. Moreover, we show that the presence of the quadratic drive term enhances the visibility of the KPZ scaling, compared to twodimensional U(1)symmetric gases, where it has remained so far elusive. Liens :[Köln Universität] 
Gwendal Feve (ENS)  Détails Fermer 
Fractional statistics of anyons in a mesoscopic collider le mardi 1er juin 2021 à 14h00 

Résumé : In threedimensional space, elementary particles are divided between fermions and bosons according to the properties of symmetry of the wave function describing the state of the system when two particles are exchanged. When exchanging two fermions, the wave function acquires a phase, φ=π. On the other hand, in the case of bosons, this phase is zero, φ=0. This difference leads to deeply distinct collective behaviors between fermions, which tend to exclude themselves, and bosons which tend to bunch together. The situation is different in twodimensional systems which can host exotic quasiparticles, called anyons, which obey intermediate quantum statistics characterized by a phase φ varying between 0 and π (1,2). For example in the fractional quantum Hall regime, obtained by applying a strong magnetic field perpendicular to a twodimensional electron gas, elementary excitations carry a fractional charge (3,4) and have been predicted to obey fractional statistics (1,2) with an exchange phase φ=π/m (where m is an odd integer). I will present how fractional statistics of anyons can be demonstrated in this system by implementing and studying anyon collisions at a beamsplitter (5). The collisions are first studied in the low magnetic field regime, where the elementary excitations are electrons which obey the usual fermionic statistics. It leads to the observation of an antibunching effect in an electron collision: electrons systematically exit in two different arms of the beamsplitter. The observed result is completely different in the fractional quantum Hall regime. Fractional statistics lead to a suppression of the antibunching effect and quasiparticles tend to bunch together in larger packets of charge in a single output of the splitter. This effect leads to the observation of negative correlations of the current fluctuations (5) in perfect agreement with recent theoretical predictions (6). (1) B. I. Halperin, Phys. Rev. Lett. 52, 1583–1586 (1984). (2) D. Arovas, J. R. Schrieffer, F. Wilczek, Phys. Rev. Lett. 53, 722–723 (1984). (3) R. de Picciotto et al., Nature 389, 162–164 (1997). (4) L. Saminadayar, D. C. Glattli, Y. Jin, B. Etienne, Phys. Rev. Lett. 79, 2526–2529 (1997) (5) H. Bartolomei, M. Kumar et al. Science 368, 173177 (2020). (6) B. Rosenow, I. P. Levkivskyi, B. I. Halperin, Phys. Rev. Lett. 116, 156802 (2016). 
Matteo Votto (LPMMC)  Détails Fermer 
Manybody scars and cluster Luttinger liquids in Rydberg atoms quantum simulators le lundi 31 mai 2021 à 10h30 

Résumé : The use of Rydberg states in order to engineer strong correlations in atomic quantum simulators has reached huge popularity in recent times, because of its flexibility in addressing different interesting regimes of quantum matter, and its push towards new interesting problems in manybody physics. An important experiment in one of these regimes showed longlived oscillations after a quench, leading to the discovery of weak ergodicity breaking in its effective hamiltonian, then resulting in the discovery of manybody quantum scars. In this seminar, stability properties under perturbation of these peculiar states are addressed with a perturbative version of the fidelity susceptibility. Furthermore, a larger class of models with constrained dynamics is considered, showing the presence of manybody quantum scars in a broader setting. In the second part of the seminar, the focus shifts to the possibilities for equilibrium physics in a novel class of Rydberg atoms quantum simulator, based on optical lattices. Similar systems have been shown to host unconventional Luttinger liquids and exotic criticality in one dimension, in their effective descriptions, known as clustering models. Since these new setups allow for dynamics on coupled chains, quantum phases of clustering models on ladders are addressed. Liens :[LPMMC] 
JeanFrançois Dayen (IPCMS, Strasbourg)  Détails Fermer 
2D0D heterostructures devices : why ? how ? what ? le mardi 25 mai 2021 à 14h00 

Résumé : Because of their atomicallythin structure, high surface to volume ratio, and reduced electric screening, new properties and functionalities are expected to emerge when exploiting the interactions of two dimensional (‘2D’) materials placed in contact with other nanomaterials such as zero dimensional (‘0D’) systems including clusters, nanocrystals and molecules. These socalled Mixeddimensional van der Waals Heterostructures are now at the forefront of basic nanoscience and applied nanotechnology, providing new sets of possibilities to tailor device functions and novel physical properties.[1] Today, I will present some of our recent achievements and ongoing works illustrating the possibilities offered by 2D0D heterostructures in various fields of nanoelectronics including singleelectron electronics [2], molecular electronics[3] and optoelectronics[4]. These 0D2D devices take advantage of the functionalities of the 0D systems (electronic, magnetic, optic…) and of the specific properties of 2D materials such as : i) van der Waals interface, ii) high diffusion of metals enabling selfordered growth of nanoclusters, iii) dual electric behavior combining inplane charge transport with outofplane electric field transparency (they are thinner than the Debye screening length), iv) exacerbated surface/interface sensitivity. The works selected for this talk will allow me to introduce some of these concepts. References : [1] D. Jariwala et al., Nat. Mater. 2017, 16, 170 ; [2] Mouafo et al, Adv. Mater. 2018, 30, 1802478; Mouafo et al., Adv.Func.Mat. 2021, 31, 2008255 ; Godel et al., Adv. Mater. 2017, 29, 1604837; [3] Noumbe et al., ACS Nano 2020, 14, 4567 ; [4] Konstantinov, J. Mat. Chem. C. 2021, 9, 2712. 
Martina Esposito (Néel)  Détails Fermer 
A reversed Kerr travelling wave parametric amplifier: from near quantumnoiselimited amplificationto microwave photonics le mardi 18 mai 2021 à 14h00 

Résumé : Travelling wave parametric amplifiers (TWPAs) recently became crucial tools in superconducting quantum technologies since they allow broadband and near quantumnoiselimited microwave detection. The main obstacle in developing TWPAs is the attainment of the phase matching condition between thepump field, which provides the energy for the amplification, and the signal field, to be amplified. I will present a new experimental approach to solving the phase matching problem by exploiting a Josephson metamaterial with insitu tunability and sign reversal of the Kerr nonlinearity: reversed Kerr phase matching. Such novel reversed Kerr TWPA (1), composed of a chain of asymmetric Josephson junctionbased inductive elements, shows amplification performance superior to the ones of previous state of the art TWPAs. In this talk, I will present the design, fabrication, and amplification characterization focusing on the novel reversed Kerr phase matching method. In addition, I will show recent experimental results on the generation of twomode squeezed statesand discuss the exciting perspective of using such a device as a source of multimode entangled states in microwave photonics. (1) Ranadive et al.A reversed Kerr traveling wave parametric amplifier(2021)http://arxiv.org/abs/2101.05815 
Vincent Michal (IRIG CEA Grenoble)  Détails Fermer 
Analog quantum simulation and quantum operations in quantum dot arrays le mardi 11 mai 2021 à 14h00 

Résumé : I would like to present advances in analog quantum simulation and quantum operations in semiconductor quantum dot arrays. The first part of the seminar is about the experimental demonstration of Nagaoka’s ferromagnetism in a quantum dot plaquette, together with its theoretical description (1). In a second part I will give a theoretical analysis of recently developed dispersively probed quantum dot microwave spectroscopy techniques in silicon (2). Then I will propose perspectives for future developments in quantum science and technology in semiconductor and hybrid platforms. References: (1) J. P. Dehollain, U. Mukhopadhyay, VPM, Y. Wang, B. Wunsch, C. Reichl, W. Wegscheider, M. S. Rudner, E. Demler, and L. M. K. Vandersypen, Nagaoka ferromagnetism observed in a quantum dot plaquette, Nature 579, 528 (2020). (2) R. Ezzouch, S. Zihlmann, VPM, J. Li, A. Aprá, B. Bertrand, L. Hutin, M. Vinet, M. Urdampilleta, T. Meunier, X. Jehl, Y.M. Niquet, M. Sanquer, S. De Franceschi, R. Maurand, Dispersively probed microwave spectroscopy of a silicon hole double quantum dot, arXiv:2012.15588. 
Leonardo Mazza (Université d'Orsay)  Détails Fermer 
One dimensional Yb gases with twobody losses: strong quantum correlations in the Zeno regime le lundi 10 mai 2021 à 11h00 

Résumé : We consider strong twobody losses in quantum gases trapped in onedimensional optical lattices, starting from the bosonic case. We exploit the separation of time scales typical of a system in the manybody quantum Zeno regime to establish a connection with the theory of the timedependent generalized Gibbs ensemble. Our main result is a simple set of rate equations that capture the simultaneous action of coherent evolution and twobody losses. This treatment gives an accurate description of the dynamics of a gas prepared in a Mott insulating state and shows that its longtime behaviour deviates significantly from meanfield analyses. The possibility of observing our predictions in an experiment with 174Yb in a metastable state is also discussed. We then move to the fermionic case, where experiments with 173Yb have already been performed: our theoretical approach pinpoints the importance of spin in determining the full dynamics of the system. 
Tobias Meng (TU Dresden)  Détails Fermer 
Weyl semimetal black holes le jeudi 6 mai 2021 à 14h00 

Résumé : Weyl semimetals derive their name from the Weyl equation, the relativistic wave equation for massless spin1/2 particles  this equation emerges as an effective lowenergy theory of certain solid state compounds. This mathematical resemblance between solid state and relativity entails a number of intriguing physical parallels. One idea that has been promoted for some time is the possibility to engineer Hamiltonians that can be mapped to spacetimes with black holes. This talks will shed some light on what that means, and use actual lattice models to explore how far this analogy carries. Going down this road will lead us to, optimistically speaking, a poor man's version of stimulated Hawking radiation. Liens : 
Andreas Kuhlmann (Basel University)  Détails Fermer 
A spin qubit in a fin fieldeffect transistor above 4K le mardi 4 mai 2021 à 14h00 

Résumé : Quantum computing's greatest challenge is scaling up. Several decades ago, classical computers faced the same problem and a single solution emerged: verylargescale integration using silicon. Today's silicon chips consist of billions of fieldeffect transistors (FinFETs) in which current flow along the finshaped channel is controlled by wraparound gates. The semiconductor industry currently employs fins of sub10 nm width, small enough for quantum applications: at low temperature, an electron or hole can be trapped under the gate and serve as a spin qubit. An attractive benefit of silicon's advantageous scaling properties is that quantum hardware and its classical control circuitry can be integrated in the same package. This, however, requires qubit operation at temperatures greater than 1 K where the cooling is sufficient to overcome the heat dissipation. Here, we demonstrate that a silicon FinFET is an excellent host for spin qubits that operate even above 4K. We achieve fast electrical control of hole spins with driving frequencies up to 150 MHz and singlequbit gate fidelities at the faulttolerance threshold. The number of spin rotations before coherence is lost at these hot temperatures already matches or exceeds values on hole spin qubits at mK temperatures. While our devices feature both industry compatibility and quality, they are fabricated in a flexible and agile way to accelerate their development. This work paves the way towards largescale integration of allelectrical and ultrafast spin qubits. 
Anastasia Gorbunova (LPMMC)  Détails Fermer 
(titre non communiqué) le lundi 3 mai 2021 à 10h30 

Liens :[LPMMC] 
Peter Rickhaus (ETH Zurich)  Détails Fermer 
Devices with Engineered Correlated States in Moire Crystals le mardi 27 avril 2021 à 14h00 

Résumé : When twisted to angles near 1 degre, graphene multilayers provide a new window on electron correlation physics by hosting gatetuneable stronglycorrelated states, including insulators, superconductors, and unusual magnets. I will report the discovery of a new member of the family(1), a correlated electronhole state, in double bilayer graphene (2,3). This state can be viewed as a density wave state and is intimately linked to equilibrium exciton condensation and superfluidity. The correlated states in moiré materials can be tuned without introducing chemical dopants, thus opening the door to a new class of devices. I will show the successful realization of an exemplary device, a gatedefined Josephson Junction in magic angle twisted bilayer graphene (4). By using multilayer gate technology we define the superconducting and insulating regions of a Josephson junction (JJ) in a singlecrystal and observe tunable DC and AC Josephson. This work is an initial step towards devices where gatedefined correlated states are connected in singlecrystal nanostructures with applications in superconducting electronics and quantum information technology. (1) P.Rickhaus, F. K. De Vries, E. Portolés, G. Zheng, T. Taniguchi, K. Wantanabe, A. H. Macdonald, T. Ihn, and K. Ensslin, ArXiv:2005.05373 (2020). (2) F. K. de Vries, J. Zhu, E. Portolés, G. Zheng, M. Masseroni, A. Kurzmann, T. Taniguchi, K. Watanabe, A. H. MacDonald, K. Ensslin, T. Ihn, and P. Rickhaus, PRL 125 (17) (3) P. Rickhaus, G. Zheng, J. L. Lado, Y. Lee, A. Kurzmann, M. Eich, R. Pisoni, C. Tong, R. Garreis, C. Gold, M. Masseroni, T. Taniguchi, K. Wantanabe, T. Ihn, and K. Ensslin, Nano Lett. 19, 8821 (2019). (4) F. K. de Vries, E. Portolés, G. Zheng, T. Taniguchi, K. Wantanabe, T. Ihn, K. Ensslin, and P. Rickhaus, arXiv:2011.00011, accepted in Nat. Nano Liens :[Peter Rickhaus][ETH Zurich] 
Aniket Rath (LPMMC)  Détails Fermer 
Importance sampling of randomized measurements for probing entanglement le lundi 26 avril 2021 à 11h00 

Résumé : We show that combining randomized measurement protocols with importance sampling allows for characterizing entanglement in significantly larger quantum systems and in a more efficient way than in previous work. A drastic reduction of statistical errors is obtained using classical techniques of machinelearning and tensor networks using partial information on the quantum state. In present experimental settings of engineered manybody quantum systems this effectively doubles the (sub)system sizes for which entanglement can be measured. In particular, we show an exponential reduction of the required number of measurements to estimate the purity of product states and GHZ states. Liens :[LPMMC] 
Williams Savero Torres (ICN2, Barcelona)  Détails Fermer 
“Spin orbit phenomena in graphenebased heterostructures le mardi 6 avril 2021 à 14h00 

Résumé : In the last years, van der Waals heterostructures have attracted an increasing attention due to their outstanding properties, resulting from combining different two dimensional materials in compact structures, that have led to the emergence of new phenomena not accesible in other platforms (1). Among them, graphene constitutes a promissing material for spintronics because it enables the transport of spin signals over larger distances compared to other systems (2). However, its low intrinsic spinorbit coupling difficults spin signal manipulation, which has prevented the fast application of graphene for spintronic devices. In this seminar, I will describe two of our recent studies performed in graphenebased heterostructures, where we demonstrate that spin signals can be generated and manipulated by means of proximity effects induced by spinorbit phenomena. In the first part, I will show how the imprinted spin texture in graphene interfaced with a transition metal dichalcogenide give rise to an anisotropic spin relaxation, where the spin lifetime for spins oriented outofplane is one order of magnitude larger than those oriented inplane (3). In the second part, I will show how such proximityinduced effects can be used to generate spin signals in graphene that can also be controlled by electrical gatting with one of the highest efficiency reported to date at room temperature (4). These results provide the building blocks for development of ultracompact devices made of two dimensional materials. (1) W. Savero Torres et al. MRS Bulletin 45(5), 357365, (2020) (2) W. Savero Torres et al. 2D Mat. 4, 041008, (2017) (3) L. A. Benítez, J.F. Sierra, W. Savero Torres et al. Nat. Phys. 14, 303308, (2018) (4) L. A. Benítez, W. Savero Torres et al. Nat. Mat. 19, 170175, (2020) Liens :[ICN2, Barcelona] 
Matthieu Dartiailh (NYU)  Détails Fermer 
Phase Signature of Topological Transition in Josephson Junctions le mardi 30 mars 2021 à 14h00 

Résumé : Topological transition transforms common superconductivity into an exotic phase of matter, which holds promise for fault tolerant quantum computing. A hallmark of this transition is the emergence of Majorana states. While two dimensional semiconductor superconductor heterostructures are desirable platforms for topological superconductivity, direct phase measurements as the fingerprint of the underlying topological transition are conspicuously missing. On gate tunable Josephson junctions made on epitaxial Al InAs, we observe a closing and a reopening of the superconducting gap with increasing in plane magnetic field. Since our junctions are embedded into a phase sensitive SQUID, we are able to measure a pi jump in the superconducting phase across the junction coincident with the closing and reopening of the superconducting gap. Theoretical simulations confirm this transition is topological and compatible with the emergence of Majorana states while the magnetic field angle dependence of the transition further constrain this scenario. Remarkably, in each junction, this topological transition can be controlled by changing the gate voltage. These findings reveal versatile two dimensional platforms for scalable topological quantum computing. 
Clemens Winkelmann (Inst. Neel)  Détails Fermer 
Heat transport and thermopower in strongly coupled single quantum dot devices le mardi 23 mars 2021 à 14h00 

Résumé : We experimentally and theoretically investigate the thermoelectric properties of gatetunable singlequantum dot junctions at milliKelvin temperatures, with particular focus on the strong tunnel coupling regime, Γ > k_BT. In the spin1/2 Kondo regime of the quantum dot, we have observed that the Seebeck coefficient displays characteristic sign changes with varying temperature and level depth, in good agreement with numerical renormalization group calculations [1]. We then move to the question of heat transport across a single quantum level. While the heat conductance of a sequentially coupled single quantum level is expected to be uniformly equal to zero, we show both experimentally and theoretically, that the inclusion of cotunnelling effects leads to restoring a finite heat conductance [2]. [1] B. Dutta et al., Nano Lett. 19, 506 (2019). [2] B. Dutta et al., Phys. Rev. Lett. 125, 237701 (2020). 
Giovanni Pecci (LPMMC)  Détails Fermer 
Spin fluctuations dynamics in harmonically trapped Fermi gases at strong interactions le lundi 22 mars 2021 à 11h00 

Résumé : We study the dynamics of a two spin component onedimensional Fermi gas trapped in an harmonic potential. We focus on the strongly interacting regime, performing a perturbative analysis starting from the regime of infinite interactions, where the model can be solved exactly. In this regime, spin and density degrees of freedom decouple and evolve independently in time. We consider an outofequilibrium initial state where the spin up and spin down particles are spatially separated in the trap. In this case, the dynamics of the particle density is trivial, while the singlespin component densities oscillate in time. We address the dynamics in the spin sector, performing numerical diagonalization of the Hamiltonian for different number of particles and comparing the different frequencies of the spin oscillations. Liens :[LPMMC] 
Michele Filippone (CEA Grenoble)  Détails Fermer 
Quantum simulation with solidstate quantum technologies : Observing manybody localization in a superconducting qubit array le mardi 16 mars 2021 à 14h00 

Résumé : In this seminar, I will discuss how quantum technologies are now able to unveil and investigate novel fundamental phenomena by simulating interacting quantum systems. In particular, we will ask : Is it possible to harness and preserve the quantum coherent properties of manybody systems?This project seems doomed to fail, as interactions in manybody systems generally lead to ergodicity, namely the inevitable loss of quantum coherence and memory about initial conditions. Nevertheless, the recent discovery of manybody localization (MBL) – a generalization of Anderson localization in the presence of interactions – has shown the possibility to circumvent ergodicity. I will illustrate an experiment in which an array of superconducting qubits probes the exotic dynamics of interacting and disordered bosons (1). Relying on realtime and interferometric probes, I will discuss how we could observe and characterize the mechanism of MBL. (1) https://arxiv.org/abs/1910.06024 
Jonathan Wise (LPMMC)  Détails Fermer 
Near field versus far field in radiative heat transfer between twodimensional metals le lundi 15 mars 2021 à 11h00 

Résumé : Using the standard fluctuational electrodynamics framework, we analytically calculate the radiative heat current between two thin metallic layers, separated by a vacuum gap. We analyse different contributions to the heat current (travelling or evanescent waves, transverse electric or magnetic polarization) and reveal the crucial qualitative role played by the dc conductivity of the metals. For poorly conducting metals, the heat current may be dominated by evanescent waves even when the separation between the layers greatly exceeds the thermal photon wavelength, and the coupling is of electrostatic nature. For wellconducting metals, the evanescent contribution dominates at separations smaller than the thermal wavelength and is mainly due to magnetostatic coupling, in agreement with earlier works on bulk metals. Liens :[LPMMC] 
Blagoje Oblak (CPhT, École polytechnique)  Détails Fermer 
Berry phases and drift in the KdV equation le lundi 1er février 2021 à 10h30 

Résumé : I consider a model of fluid particle motion closely related to the Kortewegde Vries equation governing shallow water dynamics. Using the reformulation of this model as a geodesic in an infinitedimensional group, the drift velocity of particles is shown to be an ergodic rotation number, sensitive to Berry phases produced by adiabatic spatial deformations. Along the way, I show that the topology of coadjoint orbits of wave profiles affects drift in a dramatic manner: orbits that are not homotopic to a point yield quantized rotation numbers. These arguments rely on the general structure of Euler equations, suggesting the existence of other similar applications of infinitedimensional geometry to nonlinear waves. 
Tommaso Comparin (ENSLyon)  Détails Fermer 
Quench Spectroscopy: Lowenergy excitations from realtime quantum dynamics le lundi 25 janvier 2021 à 10h30 

Résumé : Experiments with quantum simulators (made of ultracold atoms, trapped ions, etc.) are not only a way to realize lowenergy states of quantum matter, but they also offer an unprecedented set of tools to control and analyze quantum dynamics. For systems that are well isolated from their environment, such dynamics is fully determined by the manybody Hamiltonian. In our theoretical study we consider spin models with powerlaw couplings; for small systems we describe the dynamics exactly, while we employ the timedependent Variational Monte Carlo technique to treat larger sizes. We first focus on Quench Spectroscopy, an approach to characterize lowenergy excitations on top of the ground state. Starting from an uncorrelated state, spinspin correlations appear and propagate during time evolution. Their spectral analysis (in momentum and frequency) shows signatures of lowenergy excitations, like the quasiparticles described by linear spin waves. Quench Spectroscopy is an alternative to traditional spectroscopy approaches, as those implemented through inelastic neutron scattering or Bragg spectroscopy. As a second application, we look at the dynamical signatures of a subtle property of the energy spectrum, namely the presence of Anderson's tower of states. These states are connected to the eigenstates of a simpler model, consisting of a largespin rigid rotor. We show how this link can be unveiled in the time evolution of collective spin variables, and how it gives us information about the generation of squeezing during the dynamics. 
Jan Behrends (Cambridge University)  Détails Fermer 
(Super)symmetries in the SachdevYeKitaev model le vendredi 22 janvier 2021 à 11h00 

Résumé : The SachdevYeKitaev (SYK) model is an interactiononly toy model for quantum chaos, holography and nonFermi liquids. In its simplest form, Majorana fermions interact via structureless alltoall fourbody interactions. In this talk, I will demonstrate that this system belongs to one of eight (real) AltlandZirnbauer symmetry classes set only by the number of Majorana fermions. We show that, depending on the symmetry class, the system may support exact manybody zero modes. These are manifestations of an intrinsic supersymmetry that requires no relations between couplings, in contrast to existing explicitly supersymmetric extensions of the model. The supersymmetry we uncover has a natural interpretation in terms of a onedimensional topological phase supporting Sachdev–Ye–Kitaev boundary physics and has consequences away from the ground state, including in qbody dynamical correlation functions. Finally, I will briefly talk about a dynamical protocol based on Majorana zero modes that realize SYK physics. 
TudorAlexandru Petrescu (Université de Sherbrooke)  Détails Fermer 
Readout problem in circuit QED: driveinduced enhancement to the Purcell effect and other nonlinear relaxation mechanisms le lundi 18 janvier 2021 à 14h30 

Résumé : With current advances in state preparation, as well as gate and measurement operations, superconducting circuits are now a leading architecture for quantum information processing. As these systems are scaled up, strict requirements on the fidelity of operations required for computation and readout are imposed. In this talk we focus on the socalled “readout problem” in circuit quantum electrodynamics: several experiments have shown that qubit energy relaxation rates may become strongly dependent on the power of the measurement drive, even for moderate or weak drives; this hampers efforts to improve readout fidelity. To explain this, we devised a perturbation theory for drivendissipative, weakly anharmonic, superconducting circuits based on a sequence of unitary transformations. Applied to a transmon qubit coupled to a readout resonator, this approach allows us to classify the nonlinear processes that enhance qubit relaxation in the presence of resonator photons. Among these, we are able to quantify changes to the Purcell rate, and to stimulated emission. Chiefly responsible for the dressing of relaxation rates are the counterrotating terms arising from the expansion of the Josephson potential, which are usually neglected in theories based on Kerr nonlinear oscillators. Time permitting, we will discuss a general framework for quantizing driven superconducting circuits that arises from this study, with a concrete example in the accurate modeling of twoqubit gates. Liens :[Université de Sherbrooke] 
Adriano Angelone (ICTP Trieste)  Détails Fermer 
Strongly Correlated Systems of Bosons and Fermions: Manybody phenomena and Numerical Methods le lundi 11 janvier 2021 à 10h30 

Résumé : Many interesting physical phenomena are connected to strongly correlated systems, which, due to their complexity, cannot usually be studied analitically, making numerical approaches essential. The development of the latter and the study of the physical scenarios induced by strong correlations are therefore both of great importance: in my talk, I will present my results in the context of both these research directions. I will discuss my Path Integral Monte Carlo (MC) results about the equilibrium and outofequilibrium physics of a class of lattice bosonic models with extendedrange interactions, relevant for experiments with cold Rydberg and Rydbergdressed atoms, where exotic phenomena such as supersolidsupersolid transitions and (super)glass phases are induced by the formation of particle clusters in the medium and stronginteraction regime. Furthermore, I will present my work on the groundstate properties of the fermionic tJ model, a candidate Hamiltonian to describe highT_c superconductivity, in the presence of two mobile holes. Here, I employ Variational MC in conjunction with the Entangled Plaquette States (EPS) ansatz, a versatile and powerful trial wavefunction for the study of fermionic and frustrated manybody systems. My results confirm existing predictions with much higher accuracy and (unlike previously) to sizes large enough to approximate well the thermodynamic limit, and are foundational to prove the applicability of the EPS ansatz to other computationally challenging manybody problems. 