Séminaires LPMMC 2019
Philippe Jacquod (HESSO (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 

Résumé : In network theory, two issues of central importance are (i) how to assess the global robustness of a networkcoupled 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 graphtheoretic 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, centralitybased approach cannot be straightforwardly applied to deterministic networkcoupled 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 networkcoupled 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 

Annulé
Liens :Natasha Perkins 
Maxim Kharitonov (University of Würzburg)  Détails Fermer 
Universality and stability of the edge states of chiralsymmetric topological semimetals and surface states of the Luttinger semimetal le vendredi 13 décembre 2019 à 11:00 

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 chiralsymmetric boundary conditions and find that there are N et 1 universal discrete classes of them. The class determines the numbers of flatband 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 finitesize stability region of parameters of chiralasymmetric terms. This significantly extends the notion of 2D and 3D topological nodal semimetals. We demonstrate that the Luttinger model with a quadraticnode for j=3/2 electrons (Luttinger semimetal) is a 3D topological semimetal in this new sense and predict that alphaSn, 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 6 décembre 2019 à 11:00 

Résumé : In the past forty years, polyacetylene molecular wires have attracted a longstanding interest: these wires support propagation of topological domainwall states, socalled solitons, which provide a paradigm for spincharge 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 “solitondesign” in these systems. We demonstrate how to control the soliton position and how to readit 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 nonequilibrium system as a demon le vendredi 29 novembre 2019 à 11:00 

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 secondlaw of thermodynamics through the intricate action of measuring individual particles and subsequently performing feedback. Bennett (building on work of Landauer) argued that the secondlaw 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 nonequilibrium distribution to seemingly break the second law of thermodynamics. No particlebyparticle measurement or feedback is necessary. We call this an Ndemon (with the "N" for nonequilibrium), 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 secondlaw is restored by treating ``nonequilibrium'' 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 nonequilibrium 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 

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, outoftimeordered correlations, and manybody 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 

Résumé : After briefly summarizing my longterm interest in quantum physics of lowdimensional spinless fermions that attract each other, I will present our recent study of a quantum manybody lattice system of onedimensional fermions interacting with a dynamical Z2 gauge field. The gauge field mediates longrange 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 pwave 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 

Résumé : We study the ground state of a large number N of 2D anyons in an external magnetic field. We consider a scaling limit where the statistics parameter α is proportional to N^{1} when N → ∞. The model is that of bosons in a magnetic field and interacting through longrange magnetic potential generated by magnetic charges carried by each particle, smeared over discs of radius R. Our method allows to take R → 0 not too fast at the same time as N → ∞. We use the information theoretic version of the de Finetti theorem of Brandão and Harrow to justify the socalled "average field approximation": the particles behave like independent, identically distributed bosons interacting via a selfconsistent magnetic field. Liens :LPMMC 
Pierre Nataf (LPMMC, Grenoble)  Détails Fermer 
Superradiant Quantum Phase transition in Rashba Cavity QED le mardi 19 novembre 2019 à 14:00 

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 twolevel 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 spinorbit 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 : 
Leszek Sirko (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 

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/π 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 AcknowledgementsThis work was supported in part by the National Science Centre, Poland, Grant No. 2016/23/B/ST2/03979 and the Operational Programme Innovative Economy Grant No. POIG.01.01.0200008/08. J. L. was supported by the internal grant project “Introduction to quantum mechanics on graphs” of the University of Hradec Kralove.References
Liens : 
Olivier Coquand (German Aerospace Center, Cologne)  Détails Fermer 
Dynamics of granular fluids le mercredi 6 novembre 2019 à 11:00 

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 modecoupling 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 socalled mu(I) rheology known from experiments Liens : 
Srijit Goswami (TUDelft)  Détails Fermer 
Developing InSb quantum wells as a platform for topological superconductivity le mardi 5 novembre 2019 à 14:00 

Résumé : Twodimensional systems coupled to superconductors offer the opportunity to explore a variety of quantum phenomena. These include the study of novel Josephson effects , superconducting correlations in quantum (spin) Hall systems, hybrid superconducting qubits and emergent topological states in semiconductors with spinorbit interaction. In this talk I will focus on our efforts to develop InSb twodimensional electron gases (2DEGs) to engineer topological superconductivity. We show that Josephson junctions in InSb 2DEGs support supercurrent transport over several microns and display clear signatures of ballistic superconductivity. Furthermore, we exploit the large Landé gfactor and gate tunability of the junctions to control the currentphase relation, and drive transitions between the 0 and πstates. This control over the free energy landscape allows us to construct a phase diagram identifying these 0 and πregions, in agreement with theory. In addition to induced superconductivity I will discuss the operation of stable quantum confined structures (quantum dots  QDs) in these 2DEGs, which are essential elements for the manipulation and readout of topological qubits. We show that the specific material properties of InSb are strongly reflected in the transport through the QD. The small effective mass of InSb (~ 0.02 me) results in a large singleparticle confinement energy, giving rise to an evenodd variation in the size of the Coulomb diamonds, related to the spindependent filling of orbital levels in the QD. Studying the evolution of Coulomb peaks in a magnetic field allows us to determine the ground state spin configuration and directly extract the gfactor (~30). An important consequence of this large gfactor is that the ground state of the QD changes from a spinsinglet (spin zero) to spintriplet (spin one) at magnetic fields as low as 0.3 T. Together, these results establish InSb 2DEGs as a promising new platform to study topological superconductivity.
1. Ballistic superconductivity and tunable πjunctions in InSb quantum wells
C. T. Ke, C. M. Moehle et al., Nat. Commun. 10, 3764 (2019) arXiv:1902.10742
Liens : 
Nicolas Victorin (LPMMC)  Détails Fermer 
Multicomponent Gauge Dependent Quantum Gases le vendredi 18 octobre 2019 à 10:00 

Résumé : Dear Colleagues, [TITRE]The discussion will take place on Friday (18/10/2019)at 10:00 in the amphitheater of Maison des Magistères. AbstractA ladder is the simplest geometry where one can get some insight on twodimensional quantum systems subjected to a synthetic gauge field. We consider a system of bosons trapped in two onedimensional coupled lattice rings subjected to different fluxes in each leg. This specific bosonic ladder corresponds to different boundary conditions with respect to the case of a linear ladder. We explore the mesoscopic effect in this system with generic flux scheme threading the system. Effect of commensuration between flux and quantized momentum will be shown as well as fragmentation effects in the infinite interaction between boson. We will also investigate the physics of interacting excitonpolaritons in honeycomb lattice structure.The discussion will be followed by a buffet ! Chères et chers collègues, je vous invite à ma soutenance de thèse, Gaz quantiques à plusieurs composantes sous champ de jaugeL'oral aura lieu vendredi (18/10/2019)à 10h00 dans l’amphithéâtre de la Maison des Magistères. RésuméUne échelle est la géométrie la plus simple où l'on peut avoir un aperçu de la physique des systèmes quantiques à deux dimensions soumis à un champ de jauge synthétique. Nous considérons un système de bosons piégés dans un réseau formé de deux anneaux unidimensionnels couplés soumis à des flux différents. Ce système bosonique spécifique correspond à différentes conditions aux limites par rapport au cas d'une échelle linéaire. Nous explorerons les effets mésoscopiques dans ce système avec un schéma de flux générique. L'effet de la commensuration entre le flux et la quantité de mouvement quantifiée sera exposé ainsi que la fragmentation du système à interaction faible et infinie entre les bosons. Nous étudierons également la physique des excitonpolaritons dans un réseau en nid d'abeilles.La discussion sera suivie d'un buffet ! Liens : 
Riccardo Rossi (Flatiron Insitute, Smons Foundation, New York)  Détails Fermer 
New Routes Up the StronglyCorrelated Mountain le jeudi 17 octobre 2019 à 11:00 

Liens : 
Gerbold Menard (SPEC, CEA, Université Paris Saclay)  Détails Fermer 
Twoterminal conductance measurements in Majoranas SAG nanowires le mardi 15 octobre 2019 à 14:00 

Résumé : Majorana quasiparticles have generated years of intense research following the first observation of zerobias anomalies in semiconductorsuperconductor 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 zerobias 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 welldefined electrical ground without defects in the center of the wire. An alternative to these standing wires are the socalled 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 threeterminals nanowire structures allowing us to probe both sides of the same wire simultaneously [3,4]. In this presentation, I will discuss the advantage of this SAG wires and present results we obtained in these systems in relation to topological signatures as well as future possible developments using this technique.
[1] V. Mourik et al., Science 336, 1003 (2012)
Liens : 
Davide Squizzato (LPMMC)  Détails Fermer 
Exploring KardarParisiZhang universality class: from the dynamics of excitonpolariton condensates to stochastic interface growth with temporally correlated noise le mercredi 9 octobre 2019 à 14:00 

Résumé : Dear Colleagues, I am glad to invite you all to my PhD defense, Exploring KardarParisiZhang universality class: from the dynamics of excitonpolariton condensates to stochastic interface growth with temporally correlated noise. The discussion will take place on Wednesday (09/10/2019) at 14:00 in the amphitheater of Maison des Magistères. Abstract: KardarParisiZhang (KPZ) equation stands as a paradigmatic model for the investigation of OutofEquilibrium critical phenomena. In this thesis, we study realizations of the KPZ equation in two different physical systems. The first is a nonequilibrium condensate of ExcitonPolaritons, quasiparticle excitations resulting from the interaction between confined photons and excitons. A link between the dynamics of the condensate phase and the KPZ dynamics was established in the literature. Using a model and parameters close to real experimental configurations, we show that universal KPZ properties are observable in current onedimensional experimental systems, and we extend this analysis by examining the geometrydependent nature of the probability distributions of the phase of the condensate. We also show that these universal effects are robust with respect to finitesizeeffects and disorder. The second physical systems we study are classical growth surfaces whose microscopic dynamics involve temporal correlations over time. These phenomena are described by a KPZ equation in which the noise is correlated temporally. This correlation breaks one of the main symmetries of the KPZ equation and possibly leads to a new fixed point. Using the nonperturbative renormalization group (NPRG) technique, we study short and longterm temporally correlated systems in one and two dimensions. In the onedimensional case, we show that the pure fixed point KPZ persists in the shortterm and in the longterm case to a critical value of the correlation exponent. This clarifies a longstanding debate about the effects of an infinitesimal temporal correlation in the KPZ equation. In two dimensions, we find a similar image. No other result, except oneloop perturbative computation, existed in the literature for two dimensions. The defense will be followed by an italian buffet. _______________________________________________________________________________________________________________ Chères et Chers Collègues, Je suis heureux de vous inviter tous à ma soutenance de thèse, Exploration des propriétés universelles KardarParisiZhang: de la dynamique des condensats d'excitonpolariton à la croissance stochastique d'interfaces avec un bruit temporellement corrélé. La discussion aura lieu mercredi (09/10/2019) à 14h00 dans l'amphithéâtre de la Maison des Magistères. Résumé: Dans cette thèse, nous étudions des réalisations de l'équation de KardarParisiZhang (KPZ) dans deux systèmes physiques différents. Le premier est un condensat hors équilibre d’ExcitonPolaritons, des excitations quasiparticulaires issues de l'interaction entre photons confinés et excitons. Un lien entre la dynamique de la phase du condensât et la dynamique KPZ avait été établi dans la littérature. En utilisant un modèle et des paramètres proches des configurations expérimentales réelles, nous montrons que des propriétés universelles KPZ sont observables dans des systèmes expérimentaux actuels à une dimension, et nous étendant cette analyse en examinant les propriétés dépendantes de la géométrie. On montre aussi que ces propriétés ils sont robustes par rapport a des effets des taille finie et au désordre. Les deuxièmes systèmes physiques que nous étudions sont des surfaces de croissance classiques dont la dynamique microscopique implique des corrélations temporelles dans le temps. Ces phénomènes sont décrits par une équation de KPZ dans laquelle le bruit est corrélé temporellement. Cette corrélation brise l'une des symétries principales de l'équation de KPZ et conduit possiblement à un nouveau point fixe. En utilisant la technique du groupe de renormalisation non perturbartive (NPRG), nous étudions des systèmes corrélés temporellement à court et à long terme dans une et deux dimensions. Dans le cas unidimensionnel, nous montrons que le point fixe pur KPZ persiste dans le cas à court terme et dans celui à long terme jusqu’à une valeur critique de l’exposant de corrélation. Ceci clarifie un débat de longue date sur les effets d'une corrélation temporelle infinitésimale dans l’équation de KPZ. En deux dimensions, nous trouvons une image similaire. Aucun autre résultat, à l'exception d'un calcul perturbatif à une boucle, n'existait dans la littérature pour deux dimensions. La discussion serait suivie par un pot de thèse italien. Liens : 
Aleksandr Svetogorov (LPMMC)  Détails Fermer 
Quantum phase slips in onedimensional superconductors le mercredi 2 octobre 2019 à 14:00 

Résumé : Chers collègues, Quantum phase slips in onedimensional superconductorsmercredi (02/10/2019). La présentation commence à 14h00 dans l'amphithéâtre de la Maison des Magistères. La présentation sera suivie d'un pot avec des spécialités russes.
Dear colleagues, Quantum phase slips in onedimensional superconductorson Wednesday (02/10/2019).The presentation starts at 14:00 in the amphitheater of Maison des Magistères. The presentation will be followed by buffet with some Russian specialities. In this thesis quantum phase slips in onedimensional superconductors are studied. Onedimensional superconductors can be represented by two physical systems: a superconducting wire and a Josephson junction chain. A superconducting wire can be considered onedimensional, if its transverse dimensions are smaller than the superconducting coherence length. In onedimensional systems fluctuations strongly influence the system properties. The quantum phase slips correspond to quantum tunneling between different phase configurations along the superconductor. They can be of two types. Coherent quantum phase slips do not involve dissipation and only shift energy levels of the system. Incoherent quantum phase slips lead to a dissipative relaxation in the system. We start with studying an incoherent phaseslip process in a single underdamped currentbiased Josephson junction. This process corresponds to dissipative tunneling between weakly broadened levels in neighboring minima of the tilted washboard potential. We derive an expression for the voltage peaks near the resonant values of the external current, which correspond to matching energies of the lowest level in one minimum and an excited level in the lower neighboring minimum. This process is analogous to resonant Zener breakdown known for electrons in a superlattice subject to a strong electric field. We continue with studying coherent quantum phase slips in a Josephson junction chain. First, we determine the amplitude of a coherent quantum phase slip in a homogeneous chain. It has already been shown that the amplitude is determined by the imaginarytime instanton action, which can be divided into the local (corresponding to phase winding by 2π on one junction) and environmental (corresponding to phase readjustment in the rest of the chain, which is determined by gapless MooijSchön modes) parts. We derive a numerical correction to the environmental part of the action, going beyond logarithmic precision. Second, we study the effect of spatial periodic modulations of the chain parameters on the coherent quantum phase slip process. We calculate the corrections both to the local and environmental part of the coherent quantum phase slip action and show that both of them can be significant, depending on the chain and modulations parameters. Then, we study the effect of two types of quenched disorder: random spatial modulation of the junction areas and random induced background charges. The main result is that the dominant contribution to the coherent quantum phase slip action is local. We also study the statistics of the mesoscopic fluctuations of the quantum phase slips amplitude and show that it can be nonGaussian for chains which are not sufficiently long. Finally, we consider onedimensional superconducting wires. There is no microscopic theory available for the fast phase winding in the phaseslip core, where the order parameter is suppressed. However, the slow phase readjustment process, determined by the MooijSchön modes with frequencies lower than 2Δ, is analogous to that in Josephson junction chains, so the resulting environmental part of the coherent quantum phase slip action takes the same form. Therefore, we discuss how our results, obtained for Josephson junction chains, can be applied to inhomogeneous superconducting wires. Liens : 
Cyril Élouard (Université de Rochester)  Détails Fermer 
Thermodynamics in presence of quantum measurements le mercredi 25 septembre 2019 à 11:00 

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 nonzero 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 twolevel atom le mardi 24 septembre 2019 à 14:00 

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 twolevel atom driven quasiresonantly 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 steadystate which is outofequilibrium 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 : 
PierreOlivier Guimond (IQOQI, Innsbruck)  Détails Fermer 
Chiral quantum optics with atomic arrays and superconducting circuits le jeudi 19 septembre 2019 à 11:00 

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 onchip 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 onchip photon routing. In contrast to other recent proposals, our scheme does not rely on breaking timereversal 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 manybody 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 freespace with defectfree atomic arrays. 
Étienne Jussiau (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 

Résumé : English version below
Chers collègues,
J'ai le plaisir de vous inviter à ma soutenance de thèse qui aura lieu Un niveau quantique discret fortement couplé à un continuum avec une structure de bandesThèse dirigée par Robert WhitneyLa présentation (en anglais) sera suivie d'un pot où vous pourrez déguster des spécialités bourguignonnes.RésuméSuivant les progrès technologiques de la révolution industrielle, la thermodynamique classique a été développée au XIXème siècle dans le but de comprendre la conversion de la chaleur en travail intervenant dans les machines thermiques nouvellement élaborées. Les travaux de Boltzmann apportèrent une autre révolution conceptuelle avec la physique statistique. Il démontra l'origine microscopique des lois de la thermodynamique, cellesci ne décrivant en fait que le comportement macroscopique de systèmes pour lesquels la thermalisation locale est plus rapide que toutes les autres échelles de temps. Cependant, conséquemment à l'intérêt grandissant pour les nanotechnologies, il est aujourd'hui possible de manipuler des systèmes microscopiques pour lesquels la thermalisation est plus lente que les échelles de temps associés aux flux d'électrons. Une avancée technologique majeure dans ce domaine provient de l'utilisation de boîtes quantiques, des dispositifs nanométriques permettant de confiner les électrons sur des distances si petites qu'ils se répartissent sur des niveaux d'énergie discrets. Il est alors évidemment indispensable de prendre en compte les effets quantiques pour l'étude de ce type de systèmes, c'estàdire de concevoir des outils théoriques alliant thermodynamique et mécanique quantique. Les problèmes de thermodynamique quantique sont souvent abordés dans le cadre de la théorie des systèmes quantiques ouverts. L'idée générale de ce formalisme est d'étudier la dynamique d'un « petit » système quantique lorsqu'il est couplé à un autre système supposé bien plus « gros » et représentant l'environnement. On démontre alors que l'évolution temporelle du petit système peut être décrite par une équation maîtresse dans la limite où il est faiblement couplé à l'environnement. Cependant, il semble intuitivement qu'une machine pourra délivrer une puissance plus importante dans un contexte de fort couplage. Pour les problèmes de transport électronique, le formalisme de LandauerBüttiker permet de décrire le régime de fort couplage. Dans ce cadre, les électrons sont supposés ne subir que des processus de diffusion élastique dans le système central. Toutes les propriétés thermoélectriques de la machine peuvent alors être caractérisées grâce aux propriétés de transmission du diffuseur. Cependant, ce formalisme souffre aussi d'une importante limitation, la structure de bandes des réservoirs étant ignorée. Ici nous avons choisi d'adopter un point de vue différent pour aborder le régime de fort couplage en étudiant un modèle exactement soluble. Nous analysons donc le modèle de FanoAnderson décrivant un niveau discret couplé à un continuum. Nous nous intéressons particulièrement à l'influence de la densité d'états des réservoirs. On démontre en effet que, sous certaines conditions, des états liés discrets apparaissent dans les bandes interdites des réservoirs. Ces états jouent un rôle prépondérant sur la dynamique du niveau discret à temps longs : leur contribution dépend de la préparation initiale du système et peut donner lieu à des oscillations permanentes de l'occupation du niveau discret.
Nous commençons par expliciter la solution exacte du modèle en nous concentrant particulièrement son comportement à temps longs. Nous analysons ensuite deux cas particuliers. En premier lieu, nous nous intéressons aux propriétés de transport d'une boîte quantique à un niveau couplée à un semiconducteur présentant une unique bande interdite. Un état lié apparaît dans cette bande lorsque le couplage au réservoir dépasse une valeur critique ce qui affecte fortement les propriétés de transport du système. Nous étudions ensuite le cas de réservoirs décrit par un modèle de liaisons fortes dont la densité d'états ne comporte qu'une bande finie d'énergie. Nous montrons qu'un niveau discret couplé à un tel réservoir se comporte comme un système à plusieurs niveaux, sa densité d'états locale et sa fonction de transmission présentant de multiples résonances.
Dear colleagues, It is my pleasure to invite to my PhD defense which will take place on The presentation (in English) will be followed by a buffet where you will be able to taste specialities from Burgundy. A discrete quantum level strongly coupled to a continuum with a band structurePhD thesis supervised by Robert S. WhitneyAbstractFollowing the technological advances of the Industrial Revolution, classical thermodynamics was developed in the 19th century in order to understand the conversion of heat into work in newly designed machines. The works of Boltzmann brought another conceptual revolution with statistical mechanics. He demonstrated the microscopical origin of the laws of thermodynamics which actually only describe the macroscopic behaviour of systems in which local thermalization is faster than all other timescales. However, following the growing interest for nanotechnologies, it is now possible to manipulate microscopic systems in which thermalization is slower than the timescales for electron flow. A major technological advance in this field stems from the use of quantum dots, nanoscale devices which confine electrons on such small scales that they spread on discrete energy levels. It is then essential to take into account quantum effects for the study of this type of systems, that is to say to design theoretical tools combining thermodynamics and quantum mechanics. Problems of quantum thermodynamics are often tackled in the framework of the theory of open quantum systems. The general idea of this formalism is to study the dynamics of a “small” quantum system when it is coupled to another much bigger representing the environment. One can then show that the time evolution of the small system can be described by a master equation in the limit where it is weakly coupled to the environment. However, it intuitively seems that the power output of machine would be higher in the context of strong coupling. For problems of electronic transport, the LandauerBüttiker formalism allows to describe the strongcoupling regime. In this framework, electrons are assumed to solely undergo elastic scattering processes in the central system. All the thermoelectric properties of the machine can then be characterized thanks to the transmission properties of the scatterer. However, this formalism has an important limitation; it ignores the band structure of the reservoirs. Here we have chosen to adopt a different viewpoint to tackle the strongcoupling regime by studying an exactly soluble model. We therefore analyze the FanoAnderson model describing a discrete level coupled to a continuum. We are particularly interested by the influence of the reservoirs' band structure. One can indeed show that, under certain conditions, discrete bound states appear in the band gaps of the reservoirs. This state play an important rôle on the dynamics of the discrete at long times: their contribution depends on the initial preparation of the system and gives rise to persistent oscillations of the occupation of the discrete level. We start by deriving the exact solution of the model especially focusing on its longtime behaviour. We then analyze two special cases. First, we study the transport properties of a singlelevel quantum dot coupled to a semiconductor with single a band gap. A bound state appears in this gap when the coupling to the reservoir exceeds a critical value. We show that this greatly affects the transport properties of the device. We then study the case of reservoirs described by a tightbinding model which density of states consists of a single finiterange energy band. We show that a discrete level coupled to such reservoir behaves like a manylevel system as its local density of states and transmission function exhibits multiple resonances. Best regards,
Liens : 
Gernot Schaller (TU Berlin)  Détails Fermer 
A strongcoupling approach to electronic quantum transport le mardi 17 septembre 2019 à 14:00 

Résumé : The reactioncoordinate 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 strongsystem reservoir couplings and nonMarkovian 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 strongcoupling 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 nonMarkovian limits by broken thermodynamic uncertainty relations.
Papers: Liens : 
Tomas Ramos (Characterizing photonphoton interactions and correlated noise in nanophotonic systems)  Détails Fermer 
Characterizing photonphoton interactions and correlated noise in nanophotonic systems le vendredi 13 septembre 2019 à 11:00 

Résumé : We present new spectroscopic methods for characterizing correlated dephasing noise and multiphoton scattering processes in experiments with photons propagating in microwave transmission lines or optical waveguides. First, we show how correlated dephasing is manifested in the line shapes of quantum emitters, and how the same noise information contained in standard timeresolved Ramsey experiments can be extracted spectroscopically from singlephoton scattering [1,2]. Second, we present a spectroscopic method based on coherent states and homodyne detection to measure the multiphoton scattering matrix of any photonic quantum device [3]. We exemplify the protocol by reconstructing the twophoton scattering matrix of a single twolevel quantum emitter, and provide preliminary experimental results using a quantum dot inside a photonic crystal waveguide [4]. [1] T. Ramos, J.J. GarcíaRipoll, NJP 20, 105007 (2018). [2] P. Eder, T. Ramos, …, J.J. GarcíaRipoll, F. Deppe, R. Gross, Supercond. Sci. Technol. 31 115002 (2018). [3] T. Ramos, J.J. GarcíaRipoll, Phys. Rev. Lett. 119, 153601 (2017). [4] H. Le Jeannic, T. Ramos, …, J.J. GarcíaRipoll, P. Lodahl, in preparation. Liens : 
Eric Woillez (Technion)  Détails Fermer 
Is the Solar System stable? le jeudi 12 septembre 2019 à 11:00 

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 longterm 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 longterm 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 
KardarParisiZhang Equation with temporally correlated noise: a nonperturbative renormalization group approach le mercredi 11 septembre 2019 à 11:00 

Résumé : We investigate the universal behavior of the KardarParisiZhang equation with temporally correlated noise. The presence of time correlations in the microscopic noise breaks the statistical tilt symmetry, or Galilean invariance, of the original KPZ equation with deltacorrelated noise (denoted SRKPZ). Thus it is not clear whether the KPZ universality class is preserved in this case. Conflicting results exist in the literature, some advocating that it is destroyed even in the limit of infinitesimal temporal correlations, while others find that it persists up to a critical range of such correlations. Using nonperturbative and functional renormalization group techniques, we study the influence of two types of temporal correlators of the noise: a short range one with a typical timescale τ , and a powerlaw one with a varying exponent θ. We show that for the shortrange noise with any finite τ , the symmetries (the Galilean symmetry, and the timereversal one in D = 1 , 1) are dynamically restored at large scales, and the longdistance properties are governed by the SRKPZ fixed point. In the presence of a powerlaw noise, we find that the SRKPZ fixed point is still stable for θ below a critical value θc, in accordance with previous RG results, while a longrange (LR) fixedpoint controls the critical scaling for θ > θc, and we evaluate the θdependent critical exponents at this LR fixed point, in both D = 1 , 1 and D = 2 , 1 dimensions. While the results in D = 1 , 1 can be compared to previous estimates, no other prediction was available in D = 2 et 1. RéférencearXiv:1907.02256Liens :Davide SquizzatoLPMMC 
Freek Massee (LPS (ParisSud))  Détails Fermer 
Detection and manipulation of dopants and atoms in a highTc superconductor using MHz current noise le mardi 10 septembre 2019 à 14:00 

Résumé : Dopants and impurities are crucial in shaping the groundstate 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 homebuilt scanning tunnelling microscope in order to gain access to timedependent information, including shotnoise, 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 9 septembre 2019 à 11:00 

Résumé : Orbital magnetic field and strong disorder weaken superconducting correlations acting individually on a typeII swave 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 twodimensional 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 lowfrequency behavior. Our results, which emphasize the role of spatial fluctuations in the pairing amplitude, capture the nonmonotonic evolution of the magnetoresistance, consistent with experiments. We will also demonstrate that the presence of even weak disorder causes the CarolideGennesMatricon zerobias peak in vortexcore 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 LevensonFalk (University of Southern California)  Détails Fermer 
Harnessing Noise in Superconducting Quantum Circuits le mardi 3 septembre 2019 à 14:00 

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 fullscale 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 TwoDimensional Complex GinzburgLandau Equation le mercredi 28 août 2019 à 11:00 

Résumé : The complex GinzburgLandau 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 twodimensional 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 finitesize effects. The
nonzero 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 nonuniversal
aging scaling exponents. We also investigate systems with control
parameters residing in the "focusing quadrant", and identify slow
aging kinetics in that regime as well. We provide heuristic
criteria for the existence of slow coarsening dynamics and
physical aging behavior in the CGL.
Liens : 
David Rodriguez Fernandez  Détails Fermer 
Hall viscosity induced transverse voltage in twodimensional Fermi liquids le vendredi 19 juillet 2019 à 11:00 

Résumé : The absence of parity and timereversal symmetry in twodimensional Fermi liquids gives rise to nondissipative transport features characterized by the Hall viscosity. For nonvanishing 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 sliplength of our channel, while it increases with its carrier density and electronelectron 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 

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 kdesigns, which are undersamplings 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, WD and unlearnability are one and the same. Liens : 
Paolo Zanardi (University of Southern California) Annulé  Détails Fermer 
Quantum coherence in the localization transition le mercredi 10 juillet 2019 à 11:00 

Annulé
Liens : 
Pascal Simon (LPS  Orsay)  Détails Fermer 
Majorana zero modes around skyrmionic textures' le vendredi 5 juillet 2019 à 11:00 

Résumé : Recent scanning tunneling spectroscopy measurements on a superconducting monolayer of lead(Pb) with nanoscale cobalt islands, have revealed puzzling quasiparticle ingap states [1] which demand a better understanding of twodimensional superconductivity in presence of spinorbit coupling and magnetism. Thus motivated, we theoretically study a model of twodimensional swave superconductor with a fixed configuration of exchange field and spinorbit coupling terms allowed by symmetry. Using analytics and exact diagonalization of tightbinding models, we find that a vortexlike defect in the Rashba spinorbit coupling binds a single Majorana zeroenergy (midgap) state. In contrast to the case of a superconducting vortex [2], our spinorbit defect does not create a tower of ingap excitation states and our findings match the puzzling features observed in the experiment. Additionally, these properties indicate that the system realizes a pair of wellprotected 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 particlelike spin configurations that are efficiently created and manipulated. They hold great promises for nextgeneration 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 extensivelystudied 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 tightbinding 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 
Multiparticle observables from pure Yang Mills le jeudi 4 juillet 2019 à 11:00 

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, shortlived states that decay via the strong force. In this talk I will discuss progress in studying these states, by calculating multiparticle scattering observables in a the simplified framework of pure Yang Mills. This can be achieved by combining fieldtheoretic 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) threeparticles scattering amplitudes. Liens : 
Anastasia et Bastien (LPMMC)  Détails Fermer 
(titre non communiqué) le mercredi 3 juillet 2019 à 11:00 

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 & Università di Pisa)  Détails Fermer 
Fieldeffect control of the properties of InAs/InP nanowire singleelectron trnasistors le mardi 2 juillet 2019 à 14:00 

Résumé : Singleelectron 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 heterostructuredefined nanodevice. In my talk I will review our recent results on the fieldeffect 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 MatterWave Breathers le lundi 1 juillet 2019 à 10:00 

Résumé : Matterwave 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 fourfold quench of the coupling constant will generate a bisolton state whose relative solitonsoliton 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 MultiDimensional Reflection Groups le vendredi 28 juin 2019 à 11:00 

Résumé : In this presentation, I will review some of our recent successes in finding a threedimensional empirical room for the abstract multidimensional kaleidoscopes. The latter ensure solvability of the former. The areas of implementation include (a) quantum onedimensional hardcore particles with montrivial massspectra, on a line, in a box, or in a harmonic potential; (b) a quantum onedimensional 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 amplifier”, (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 onedimensional chemical processes and the usage thereof for miniaturization of chipbased atom interferometers, and (c) nineteen threeparametric families of solvable electrostatic problems in piecewisespherical cavities with conducting grounded walls. Liens : 
Ambroise van Reokeghem (CEA Grenoble)  Détails Fermer 
Transitionmetal pnictides : electrons and phonons le mercredi 26 juin 2019 à 11:00 

Résumé : The family of ironbased 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 ironbased compounds. Liens : 
Zhihui Peng (Hunan Normal University, Changsha, China )  Détails Fermer 
Coupling of a Cavity and a Transmission Line with a Superconducting Artiﬁcial Atom le mardi 25 juin 2019 à 14:00 

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 vacuuminduced AulterTownes splitting[1] which has potential application in microwave quantum network. We also observed anomalous resonance fluorescence of an atomcavity coupled system[2] which is qualitatively different from the drivenatom 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 (ENSLyon)  Détails Fermer 
Assessing manybody quantumness via correlation functions le vendredi 21 juin 2019 à 11:00 

Résumé : Decades of research into the foundational aspects of quantum mechanics — started with the EinsteinPodolskyRosen (EPR) paradox on entanglement and nonlocality — 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 nonclassical nature of quantum states, namely their characterization as coherent superpositions featuring nonlocal quantum correlations. This question becomes particularly intriguing and intricate when moving to manybody systems: the exponential growth of quantum information with the system size makes manybody 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 quantumclassical or micromacro boundary; but it has also immediate bonuses, since assessing quantumness of manybody 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 manybody 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 nonclassicality, 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, Tomaso et Anatole (LPMMC)  Détails Fermer 
Présentations des stages de M2 le mercredi 19 juin 2019 à 11:00 

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 

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 quasi1D 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 

Résumé : Conventionally, the quantum oscillations of magnetisation [the de Haasvan 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 selfconsistent 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 µSQUIDs for probing nanomagnetism le mardi 11 juin 2019 à 14:00 

Résumé : Magnetometry using micronsize superconducting quantum interference devices (µSQUIDs) has been remarkably successful in probing classical as well as quantum regimes of magnetism in single nanoparticles. This technique can be further improved for higher speed and sensitivity with hysteresisfree µSQUIDs. This is difficult, particularly, at low temperatures, which is essential for probing quantummagnetism. The hysteresis in these devices arises from thermal instabilities in superconducting weaklinks and neighboring region. The heat generated in resistive normal region gives rise to a self sustained hotspot. This leads to two possible states, hot (normal) and cold (superconducting), and hence bistability. Such hotspot and hysteresis has been modeled in the past by using steady state thermal heat balance equations. However, as we approach the hysteresisfree 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 nonhysteretic regimes. Slow relaxation of quasiparticles, which are generated due to phase dynamics, is found to be a bottleneck, which is the case for several superconducting devices including SINcoolers 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 µSQUIDs. This new model also reveals an interesting nonlinear 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 nanoneedles by using these optimized nonhysteretic µSQUIDs. Liens : 
Peter Makk (University of Basel & Budapest University of Technology and Economics,)  Détails Fermer 
Engineering exotic states in graphene heterostructures le mardi 4 juin 2019 à 14:00 

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 nonequilibrium 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 

Résumé : The investigation of the propagation of light in mesoscopic, i.e. often micrometerscale, systems is a rich subject providing insights ranging from quantum chaos in open systems to new schemes for realizing microlasers. The concept of quantumclassical, here waveray, 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 raypicture 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 farfield emission characteristics of optical microcavities. The propagation of electromagnetic waves in threedimensional optical microcavities requires to pay attention to the evolution of the light's polarization as a new degree of freedom. In systems like dielectric Möbiusstrips or coneshaped microtube cavities, the polarization state of resonant whispering gallerytype 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 spinorbit interaction of light and their interpretation in terms of Berry phases, and relevant for potential applications. Liens :Martina Hentschel 
Michael Urban (IPN Orsay)  Détails Fermer 
Superfluidity in the inner crust of neutron stars le vendredi 24 mai 2019 à 11:00 

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 neutronrich 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 manybody theory, mostly because of mediumpolarisation 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 
Inflight manipulation of single electrons le mardi 21 mai 2019 à 14:00 

Résumé : Over the past decade, an important effort has been made in the field of lowdimensional electronic conductors towards singleelectron electronics with the goal of gaining coherent control over single flying electrons in solidstate devices [1]. In this talk I will present our recent advances towards the realization of electronic flyingqubit 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 timeresolved measurements of ultrashort singleelectron charge pulses injected into a quasionedimensional quantum conductor. We show that the velocity of such a singleelectron pulse is found to be much faster than the Fermi velocity due to the presence of strong electronelectron interactions and can be tuned over more than an order of magnitude by electrostatic confinement. In addition, our setup allows us to tune our system continuously from a clean onechannel TomonagaLuttinger liquid to a multichannel Fermi liquid [2]. Our results are in quantitative agreement with a parameterfree theory and demonstrate a powerful new probe for directly investigating realtime dynamics of fractionalisation phenomena in lowdimensional conductors. In the second part of the talk, I will concentrate on SAWassisted singleelectron transport. I will present our recent results on highlyefficient electron routing in a beamsplitter 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 ondemand 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 solidstate 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 

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 

Résumé : I shall discuss the homogeneous Bose gas at positive temperatures within Bogoliubov theory. The theory arises by restricting the Hilbert space to quasifree 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 

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 twoelectron HongOuMandel 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 
Transitionmetal pnictides: electrons and phonons le mardi 30 avril 2019 à 11:00 

Liens : 
Gabriel Apolinario (Université fédérale de Rio, Brésil)  Détails Fermer 
Onset of intermittency in stochastic Burgers hydrodynamics le mardi 16 avril 2019 à 11:00 

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. 
Arnaud Tourin (Institut Langevin)  Détails Fermer 
Acoustic bubbly metamaterials: subwalength focusing, negative refraction and superabsorption le vendredi 12 avril 2019 à 11:00 

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 lowfrequency 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 multiplescattering 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 
Densitywave steadystate phase of dissipative ultracold fermions with nearestneighbor interactions le mercredi 10 avril 2019 à 13:30 

Résumé : I will describe our recent results about the effect of local dissipation on densitywave ordering in the extended FermiHubbard model with both local and nearestneighbor interactions. For this purpose, we used a recent variant of nonequilibrium dynamical meanfield theory with the auxiliary master equation approach which allows to treat nonperturbatively both local dissipation and local interaction. I will show how densitywave 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 highlyexcited 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 

Résumé : Lightmatter 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 storageandretrieval 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 dipoledipole 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 lightmatter 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 longrange dipoledipole coupling between atoms not only via the free space, but also through the waveguide mode. Liens : 
Michael Hatridge  Détails Fermer 
ANNULÉ le mardi 9 avril 2019 à 14:00 

Résumé : ANNULÉ Liens : 
Julien Toulouse  Détails Fermer 
Rigorous combination of wavefunction methods and densityfunctional theory for electronicstructure calculations le vendredi 5 avril 2019 à 11:00 

Résumé : I will first give a brief overview of the main goals of quantum chemistry and of the two families of electronicstructure computational methods used to solve the manyelectron Schrödinger equation in this domain, namely wavefunction methods and densityfunctional 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 electronelectron interaction into longrange and shortrange contributions. The idea is to use a manybody wavefunction method for the longrange contribution, coupled with a densityfunctional approximation for the shortrange contribution. I will show two specific realizations of this rangeseparated wavefunction/densityfunctional theory using for the wavefunction method: 1) a randomphase approximation, which allows us to describe van der Waals intermolecular interactions; 2) a selected configurationinteraction 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 3 avril 2019 à 11:00 

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 outofequilibrium. 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 

Résumé : Quantum mechanics postulates that a measurement forces the wavefunction 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 wellaccustomed 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 backaction. 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 backaction.
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 gfactor and spinorbit interaction, leading to peculiar behaviour of the Josephson supercurrent. Specifically, the switching current exhibits nonmonotonic 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 Josephsonphi0 junction[2]. Such phi0junctions can serve as smoking gun signatures of Majorana fermions.
Liens : 
Xavier Montiel (University of London)  Détails Fermer 
Generation of pure superconducting spin current in superconducting heterostructures via nonlocally induced magnetism le mercredi 20 mars 2019 à 13:30 

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 spincurrents carried by equalspin triplet Cooper pairs in a superconductor (S) sandwiched between a ferromagnet (F) and a normal metal (Nso) with intrinsic spinorbit coupling [2]. I will show that in the presence of Fermiliquid interactions, the superconducting proximity effect can induce nonlocally a ferromagnetic exchange field in the normal layer, which disappears above the superconducting transition temperature of the structure. The internal Fermiliquid exchange field leads to the onset of a spin supercurrent associated with the generation of longrange spintriplet 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 Fermiliquid 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 nonequilibrium spin currents in superconducting structures.
Liens : 
Benjamin Lenz (Centre de physique théorique, École polytechnique)  Détails Fermer 
Effects of nonlocal correlations on spectral properties of doped Sr2IrO4 le mercredi 20 mars 2019 à 11:00 

Résumé : The spinorbit Mott insulator Sr_{2}IrO_{4} has been in the spotlight in recent years due to its striking similarities to isostructural highT_{c} 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 holedoping by means of a combined abinitio electronic structure and oriented cluster dynamical meanfield approach. Within this treatment, important ingredients like spinorbit coupling and distortions of the oxygen octahedra as well as Hubbard interactions and nonlocal charge fluctuations are taken into account. The calculated spectral function of pure Sr_{2}IrO_{4} compares well with angularresolved photoemission measurements, both in the lowtemperature antiferromagnetic and hightemperature paramagnetic phase, and allows to study emerging changes under electron and holedoping. Special emphasis of my talk will be placed on pseudogap features of the momentumresolved spectral function of electrondoped Sr_{2}IrO_{4}, which are found to be in good agreement with experiment. Liens :Benjamin LenzCentre de physique théorique, École polytechnique 
Oleksandr Tsyplyatyev (Francfort University)  Détails Fermer 
A hierarchy of strongly correlated modes in quantum wires le vendredi 15 mars 2019 à 11:00 

Résumé : The natural excitations of an interacting onedimensional 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 manybody 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 energymomentum planes. For example, in the spectral function the firstlevel (strongest) excitations form a mode with parabolic dispersion, like that of a renormalised single particle. The secondlevel excitations produce a singular powerlaw line shape to the firstlevel mode and multiple powerlaws 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 doublewell heterostructures. The momentumresolved tunnelling in this setup directly probes the spectral function of electrons at all energy scales giving access to the spincharge 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 

Résumé : Je discuterai l'existence d'une branche collective de "Higgs" dans les condensats fermioniques, gaz ultrafroids ou supraconducteurs. Cette question, qui bénéficie désormais de plusieurs résultats expérimentaux, demeure controversée d'un point de vue théorique du fait de la présence d'un continuum (lié à la brisure des paires de Cooper) aux énergies où la branche collective devrait se trouver. En particulier, à vecteur d'onde nul, il n'est pas certain qu'il existe un mode collectif clairement distinct du bord du continuum à 2Δ (deux fois la bande interdite). À vecteur d'onde petit mais non nul, plusieurs prédictions contradictoires de la relation de dispersion coexistent dans la littérature. Nous montrons que la branche existe bel et bien à potentiel chimique μ positif et à vecteur d'onde strictement positif et inférieur à sqrt(2m*μ)/ℏ. Pour cela, nous traitons le couplage au continuum de façon non perturbative en faisant un prolongement analytique à travers la ligne de coupure qui lui est associée dans la fonction de Green du champ de paire. Nous obtenons ainsi la relation de dispersion du mode collectif quel que soit le vecteur d'onde et la force des interactions. Enfin, nous montrons que la résonance associée au mode collectif est bien visible dans la fonction de réponse du champ de paires. Liens : 
Hélène le Sueur (CSNSM, Univ. ParisSud)  Détails Fermer 
Microscopic charged fluctuators as a limit to the coherence of disordered superconductor devices le mardi 12 mars 2019 à 14:00 

Résumé : By performing experiments with thinfilm 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 outofequilibrium 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 solidstate 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 condensedmatter systems with light le mercredi 6 mars 2019 à 11:00 

Résumé : Using strong lightmatter interactions to control the quantum properties of condensedmatter
systems is an ongoing broad research effort. In particular, it is known that transport
[1,2] and superconductivity [3,4] can be modified using an external radiation. In the
absence of the latter, it is an interesting question whether these properties may be also
affected by coupling the relevant matter excitations to the vacuum field of a cavitytype
structure. In this talk, I will present different systems and regimes where this idea can be
exploited. Particularly interesting is the socalled “ultrastrong” lightmatter coupling regime
[5], which occurs when the coupling strength is comparable to the bare transition
frequency. I will first show that this regime can be achieved in quantum Hall systems
coupled to a terahertz resonator [6,7], which can affect the magnetotransport properties of
the electron gas [8,9]. I will also explain how quantum Hall systems embedded in a
photonic band gap material can be used to reach an interesting regime featuring very large
cooperativities [10,11]. A simple model to study the interplay between charge transport and
lightmatter interactions in a chain of twolevel systems will then be presented. Using nonequilibrium
Green’s functions and quantum master equations, I will show that in the
dissipative regime where the cavity photon decay rate is the largest parameter, the lightmatter
coupling is responsible for a steadystate current enhancement scaling with the
cooperativity [12,13]. Possible applications of these results will be also discussed. Finally, I
will present some recent theoretical results showing that ultrastrong coupling without
external resonators can be achieved in certain metallic crystals where plasmons coexist
with phonons exhibiting large oscillator strengths. These strong lightmatter interactions
give rise to intrinsic surface plasmonphonon polaritons, which offer the unique possibility
to control the phonon properties by tuning the electron density and the crystal thickness
[14]. In particular, these surface polaritons can lead to large enhancements of the electronphonon
scattering, which could have a profound inuence, e.g. on the superconducting
properties of certain crystals. I will conclude by giving some perspectives of this work.
References[1] M. A. Zudov et al. Phys. Rev B 64, 201311(R) (2001)[2] R. Mani et al. Nature 420, 646 (2002) [3] A. F. G. Wyatt et al., Phys. Rev. Lett. 16, 1166 (1966) [4] D. Fausti et al., Science 331, 189 (2011) [5] C. Ciuti, G. Bastard, and I. Carusotto, Phys. Rev. B 72, 115303 (2005) [6] D. Hagenmüller, S. De Liberato, and C. Ciuti, Phys. Rev. B 81, 235303 (2010) [7] G. Scalari et al., Science 335, 1323 (2012) [8] N. Bartolo and C. Ciuti, Phys. Rev. B 98, 205301 (2018) [9] G. L. ParaviciniBagliani et al., Nat. Phys. 15, 186 (2019) [10] D. Hagenmüller, Phys. Rev. B 93, 235309 (2016) [11] Q. Zhang et al., Nat. Phys. 12, 1005 (2016) [12] D. Hagenmüller et al., Phys. Rev. Lett. 119, 223601 (2017) [13] D. Hagenmüller et al., Phys. Rev. B 97, 205303 (2018) [14] D. Hagenmüller et al., arXiv:1810.10190 (2018) Liens : 
Boris Brun (UCLouvain, Belgium)  Détails Fermer 
Imaging thermoelectric transport through quantum nanostructures le mardi 5 mars 2019 à 14:00 

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 electronelectron interactions give rise to conductance [2,3] and thermoeletcric [4] anomalies. By scanning the polarized tip in front of the QPC, we create a FabryPérot cavity between the QPC channel and the tipdepleted 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 π 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, 183186 (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 
Benjamin Wandelt (IAP and ILP, Sorbonne University, Paris, CCA, Flatiron Institute, New York)  Détails Fermer 
Confronting Theory and Data in Cosmology le vendredi 1 mars 2019 à 11:00 

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 GrossPitaevskii regime le vendredi 15 février 2019 à 11:00 

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 (GrossPitaevskii 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 resonantlydriven polaritons superfluids le mercredi 13 février 2019 à 13:30 

Résumé : Excitonpolaritons, microcavity halfmatter halflight quasiparticles, when resonantly driven exhibit a superfluid regime. Accordingly, topological excitations similar to those predicted in equilibrium superfluids may spontaneously appear [1,2]. However, the nonequilibrium 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 nonlinear 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 

Résumé : Secondorder 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 finitetemperature phase diagram (the socalled 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 counterintuitive, genuinely quantum, mechanism, for the suppression of certain fluctuations at the QCP of the quantum Ising model (leading to spinsqueezing). Liens :Irénée Frérot 
Sofian Teber (Jussieu) Annulé  Détails Fermer 
Radiative corrections in planar Dirac liquids le vendredi 8 février 2019 à 11:15 

Annulé
Liens : 
Antonin Coutant  Détails Fermer 
Black holes in fluid flows le mercredi 6 février 2019 à 13:30 

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 : 
PierreÉlie Larré (Université de CergyPontoise)  Détails Fermer 
Quantum manybody physics with nonlinear propagating light le mercredi 6 février 2019 à 11:00 

Résumé : The propagation of a paraxial and quasimonochromatic quantum light field in a dispersive and nonlinear dielectric is considered. In this alloptical setup, the space propagation of the field's envelope can be mapped onto the time evolution of a quantum fluid of interacting photons. The resulting quantum manybody system constitutes a particular class of quantum fluids of light and presently attracts growing interest as a platform for quantum simulation. I will review recent theoretical and experimental progresses in this rapidly emerging research field, including investigations on superfluidity, elementary excitations, quantum quenches (possibly in the presence of disorder), thermalization, BoseEinstein condensation, and topology.
Liens : 
David Di Vincenzo (Peter Grünberg Institute)  Détails Fermer 
Precision couplings and tailored couplings for highfidelity quantum computing le vendredi 1 février 2019 à 11:00 

Résumé : This is a twopart talk, one is about consideration of longitudinal rather than transverse coupling for qubitresonator 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 manybody systems le mercredi 30 janvier 2019 à 13:30 

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 lowdimensional (mainly onebody) systems. There is a growing interest in transferring these techniques towards manybody systems. First I will explain how to describe the energy spectrum and the associated eigenstates for the seminal BoseHubbard 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 
Manybody localization and thermalization in onedimensional quantum systems le mercredi 30 janvier 2019 à 11:00 

Résumé : At high energy, isolated quantum systems generically thermalize, their macroscopic observables obeying the classical laws of thermodynamics. In manybody localized systems however, transport is prevented, effectively breaking thermalization. The phenomenon of manybody 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 outofequilibrium open systems.
Selected references
Liens : 
Dmitry Bagrets (University of Cologne)  Détails Fermer 
The SachdevYeKitaev model, its holographic dual and quantum conformal fluctuations le vendredi 25 janvier 2019 à 11:00 

Résumé : The fascinating SachdevYeKitaev (SYK) model describing a large number of randomly interacting Majorana fermions represents an ultimate example of the AdS/CFT correspondence in 1 et 1 spacetime 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 infrared. As such, the soft Goldstone mode in the spectrum of the model emerges which is known to be described by the socalled 'Schwarzian' action. In my talk, after a general exposition to the SYK model, I will concentrate on its quantum deeply infrared 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 longtime limit of 2 and 4point correlation functions of Majoranas. The range of new results predicted by such mapping encompasses universal powerlaw 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 
Rangeseparated DFT for molecular excitation energies le mercredi 23 janvier 2019 à 13:30 

Liens : 
Giovanni Martone (LPTMS, Orsay)  Détails Fermer 
Static and dynamic properties of spinorbitcoupled Bose gases le mercredi 23 janvier 2019 à 11:00 

Résumé : The realization of synthetic spinorbit 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 twocomponent BoseEinstein condensates with equalweighted Rashba and Dresselhaus spinorbit couplings. Their phase diagram includes different structures, such as a spinpolarized planewave 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 longsought 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 

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 : 