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# Séminaires généraux

## Modalités d’organisation

Quand ? Le mercredi à 13h45, une semaine sur deux, toute l’année sauf vacances d’été et de Noël, ainsi que pendant le cours au Collège de France. Il sera précédé à 13h30 par un café.

Où ?  Collège de France (Octobre-Décembre), École Normale Supérieure (Janvier-Avril) et Université Pierre et Marie Curie (Mai-Juillet).

Comment être informé ? Outre la traditionnelle diffusion par email, les séminaires seront annoncés sur le calendrier de l’intranet, sur le site public du LKB ainsi que sur Semparis (service des séminaires parisiens) et sur le site du département de physique de l’ENS.

## Liste des séminaires

 Date Speaker Title and abstract Time and location 19/07/2018 E WU (East China Normal University, Shanghai) High-efficiency broadband single-photon frequency upconversion Nowadays, single-photon frequency upconversion detectors (UCDs) have recently drawn a great deal of attention because it can be utilized as a quantum interface that enable qubits to transfer from infrared to visible regime, while preserving the quantum state information and then use Si-APDs to count the visible sum-frequency replicas of the infrared photons with high detection efficiency, high signal-to-noise ratio, short dead time, low timing jitter. The UCDs are increasingly used in more fields of importance, such as quantum key distribution (QKD), quantum metrology, quantum computation and quantum tomography. However, when pulse duration of the signal photons are very short to femtosecond where the spectrum of signal photons is very broadband to fulfill the Fourier transformation, the conversion efficiency will be much decreased, because the spectral width from these source is much wider than the acceptance bandwidth of the PPLN waveguide (0.2 nm). Here, we demonstrated a high efficiency telecom wavelength broadband single-photon frequency upconversion in PPLN crystal. By optimizing the pump light spectral bandwidth, we got 19.54 % conversion efficiency with a signal spectral bandwidth of 7.9 nm. 13:45, Jussieu tour 13-23 2ème étage. 28/06/2018 Lincoln Carr (Colorado School of Mines) Many-body quantum chaos of ultracold atoms in a quantum ratchet There are now over 200 quantum simulators on at least 8 separate architectures with long coherence times and controlled dynamics. These experimental systems have generated tremendous excitement about driven interacting quantum systems resulting in physics ranging from time crystals to dynamical many-body localization. The quantum ratchet adds a new feature to periodic driving: a preferred direction in both time and space, i.e., parity and time-reversal symmetry-breaking. By studying weakly interacting ultracold bosons in a quantum ratchet on a ring in position, momentum, and Floquet representations, we demonstrate the limits of known measures of quantum chaos in a system with a clearly defined and rather famous semiclassical or mean-field limit, and moreover supporting experiments. We show that the usual Wigner-Dyson statistics used to identify chaos are smeared out as we couple non-resonant modes into the drive. In contrast, the entropy of entanglement, condensate depletion, and inverse participation ratio all serve as accurate alternate identifiers for the chaotic regime in which the current on the ring flip-flops with a positive Lyapunov exponent in the mean-field limit. The dimension of the strange attractor is found to depend on the local vs. global nature of the observables. Moreover, the growth of depletion indicates mean field theory breaks down at realistic experimental times scaling polynomially as in the chaotic regime. This study opens the door to beyond single-frequency many-body Floquet physics showing many surprises and subtleties in both the quantum many-body dynamics and the mean-field limit (or lack thereof). Our prediction of a concrete time at which depletion grows is experimentally observable via an interference experiment. The dynamics and emergent structure of higher order correlators remains an especially intriguing avenue of exploration as we find that, contrary to oft-stated popular opinion, chaos, at least in the quantum ratchet, does not lead to high entanglement. 13:45, Jussieu tour 13-23 2ème étage. 21/06/2018 Jook Walraven (Univ. of Amsterdam) Ultrafast many-body interferometry of impurities coupled to a Fermi sea The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions. 13:45, Collège de France, salle 5 17/05/2018 Aephraim Steinberg (Uni. of Toronto) Comment compter un photon unique, et trouver un résultat de 1000 : d’étranges conséquences de la mesure quantique, à l’aide d’atomes froids Je présenterai nos travaux expérimentaux concernant les mesures quantiques faites sur des systèmes post-sélectionnés. Notamment, nous nous servons d’atomes froids et de l’effet «EIT» pour créer des interactions photon-photon suffisamment puissantes qu’on observe la phase nonlinéaire imprimée par des impulsions au niveau du photon unique sur un fasiceau sonde. En s’appuyant sur le formalisme de «weak measurement» de Yakir Aharonov et al., nous montrons aussi que quand la mesure est effectuée sur un photon post-sélectionné dans un état final donné, l’effet sur la sonde peut être «amplifié», de sorte qu’une mesure du nombre de photons dans une région d’espace peut même dépasser le nombre de photons total. Je parlerai des questions à la fois philosophiques et pratiques qui se posent autour de ce genre d’observation, et si le temps le permet, je dirai quelques mots à propos d’autres manips en cours, telle une expérience pour mesurer la durée de la traversée d’une barrière à effet tunnel par les atoms ultrafroids (~900 pK), et une application des idées venues de l’informatique quantique à des problèmes d’imagerie classique (super-résolution). 13:45, Jussieu, salle 210 couloir 13-23 04/04/2018 Aaron Tranter (ANU, Australia) Deep learning cold atomic ensembles for quantum memories Quantum memories are integral to the realization of quantum information networks and quantum information processing. A promising platform is gradient echo memory (GEM) in cold atomic systems with demonstrated efficiencies of ~87%. We demonstrate the first application of a deep learning algorithm to a cold atomic system in order to increase the optical depth (OD) of our atomic trap and thus increase memory efficiency. We perform a 63 parameter optimisation and find solutions that are agnostic to considerations regarding monotonicity or continuity and vastly outperform human solutions increasing our optical depth by (81+-3)%. We also observe a physical change in the atomic cloud corresponding to the spatial distribution of the atomic ensemble and apply the optimisation to the GEM protocol. 13:45, Jussieu, salle 210 couloir 13-23 21/03/2018 Denis Vasilyev (IQOQI Innsbruck) A Quantum Scanning Microscope for Cold Atoms and an overview of a ‘Few-Atom’ Quantum Optical Antenna I will present my recent work made in IQOQI. The main part of the talk is devoted to the quantum scanning microscope arXiv:1709.01530 (to be published in PRL) We propose and analyze a scanning microscope to monitor live’ the quantum dynamics of cold atoms in a Cavity QED setup. The microscope measures the atomic density with subwavelength resolution via dispersive couplings to a cavity and homodyne detection within the framework of continuous measurement theory. We analyze two modes of operation. First, for a fixed focal point the microscope records the wave packet dynamics of atoms with time resolution set by the cavity lifetime. Second, a spatial scan of the microscope acts to map out the spatial density of stationary quantum states. Remarkably, in the latter case, for a good cavity limit, the microscope becomes an effective quantum non-demolition (QND) device, such that the spatial distribution of motional eigenstates can be measured back-action free in single scans, as an emergent QND measurement. In the final part of the talk I will present an overview of our ongoing work involving cold Rydberg atoms in regular arrays forming an optical antenna arXiv:1802.05592 We describe the design of an artificial free space’ 1D-atom for quantum optics, where we implement an effective two-level atom in a 3D optical environment with a chiral light-atom interface, i.e. absorption and spontaneous emission of light is essentially unidirectional. This is achieved by coupling the atom of interest in a laser-assisted process to a few-atom’ array of emitters with subwavelength spacing, which acts as a phased-array optical antenna. We develop a general quantum optical model based on Wigner-Weisskopf theory, and quantify the directionality of spontaneous emission in terms of a Purcell $\beta$-factor for a given Gaussian (paraxial) mode of the radiation field, predicting values rapidly approaching unity for few-atom’ antennas in bi- and multilayer configurations. Our setup has for neutral atoms a natural implementation with laser-assisted Rydberg interactions, and we present a study of directionality of emission from a string of trapped ions with superwavelength spacing. 13:45, ENS, salle Conf IV 07/02/2018 Nouveaux doctorants et postdocs (LKB) Présentations des nouveaux arrivants du LKB Jour II 14:00, CdF, salle 5 13/12/2017 Igor Mekhov (SPEC CEA, St. Petersburg State University, University of Oxford) Weak measurements and quantum optical lattices for strongly correlated bosons and fermions While optical lattices are well-established systems, the quantum nature of light is neglected in all setups so far. We show theoretically that the light quantization significantly broadens the range of physical phenomena. We prove that the quantum backaction of weak global measurement constitutes a novel source of competitions in many-body systems. This leads to novel effects beyond physics of open dissipative systems: multimode oscillations of macroscopic superposition states, protection and break-up of fermion pairs, as well as generation of antiferromagnetic states. Novel processes beyond standard Hubbard models can be designed by the measurement, entering the field of non-Hermitian many-body physics: long-range correlated pair tunnelling and Raman-like second-order transitions beyond typical quantum Zeno dynamics. We demonstrate the generation of multipartite mode entanglement and feedback control of many-body states out of reach of dissipative phase transitions.Quantization of optical lattice potentials enables quantum simulations of various long-range interacting systems unobtainable using classical optical lattices. This leads to new quantum phases (dimers, trimers, etc. of matter waves similar to valence bond solids) different from density orders (e.g. supersolids and density waves) directly benefiting from the collective light-matter interaction. 13:45, ENS, conf IV 17/01/2018 Nouveaux doctorants et postdocs (LKB) Présentations des nouveaux arrivants du LKB Jour I 14:00, CdF, salle 2 13/12/2017 Mark Kasevich (Stanford) Quantum measurement strategies for atoms, photons and electrons Quantum measurement protocols based on dispersive cavity-assisted interactions will be described. We will show how these protocols lead to performance improvements for precision atomic sensors and to new tests of quantum mechanics. We will also describe quantum imaging methods based on repeated coherent interactions in degenerate optical and electron cavities. 13:45, CdF, salle 2 06/12/2017 Serge Reynaud (LKB) with collaboration MICROSCOPE Premiers résultats de MICROSCOPE Les premiers résultats du satellite MICROSCOPE démontrent avec une précision améliorée que les corps tombent de façon universelle dans le vide. Il s’agit d’une nouvelle confirmation de la relativité générale proposée par Albert Einstein il y a plus d’un siècle et qui a été encore vérifiée récemment par la détection des ondes gravitationnelles. MICROSCOPE (Microsatellite à trainée compensée pour l’observation du principe d’équivalence) a été lancé le 25 avril 2016. L’analyse des premières mesures scientifiques effectuées améliore la précision du test du principe d’équivalence au niveau inégalé de 2.10-14. Ce résultat repousse les limites d’une éventuelle violation du principe d’équivalence d’un facteur 10 et apporte de nouvelles contraintes aux théories d’extension de la relativité générale. Les autres données déjà acquises, ou à collecter d’ici la fin de la mission au printemps 2018, amélioreront encore cette précision pour se rapprocher de l’objectif de 10-15.The MICROSCOPE mission: first results of a space test of the Equivalence Principle Pierre Touboul et al. Phys. Rev. Lett. 119, 231101 (2017) 13:45, ENS, salle L363/L365 22/11/2017 Dmitry Petrov (LPTMS, Univ Paris-Sud) Mesoscopic few-body problem with short-range interactions I will describe our work with Betzalel Bazak on the N+1-body fermionic problem in three dimensions [1] and N-boson problem in two dimensions [2]. By developing a new method of solving few-body integral equations we are able to obtain precise results for ground state energies. In particular, we predict a universal pentamer state and five-body Efimov effect for the 4+1 fermionic problem and quantify the few-to-many-body crossover for two-dimensional bosons. I point to Refs. [3] and [4] as a recommended reading. [1] B. Bazak and D.S. Petrov, “Five-body Efimov effect and universal pentamer in fermionic mixtures”, Phys. Rev. Lett. 118, 083002 (2017)  [2] B. Bazak and D.S. Petrov, “Energy of N two-dimensional bosons with zero-range interactions”, arXiv:1711.02345  [3] Y. Castin, C. Mora, and L. Pricoupenko, “Four-Body Efimov Effect for Three Fermions and a Lighter Particle”, Phys. Rev. Lett. 105, 223201 (2010)  [4] H.-W. Hammer and D. T. Son, “Universal Properties of Two-Dimensional Boson Droplets”, Phys. Rev. Lett. 93, 250408 (2004) 13:45, CdF, salle 2 25/10/2017 Juan Pablo Paz (Université de Buenos Aires) Using a Quantum Work Meter to test non equilibrium fluctuation theorems The so called “fluctuation theorems” are one of the most important results in non equilibrium thermodynamics obtained during the last decades. They connect the behavior of a system evolving far away from equilibrium with certain properties of thermal equilibrium states of the same system. The work probability distribution plays a major role in such theorems and its measurement attracted a lot of interest recently. In this talk I will review a new strategy for measuring work on a quantum system which evolves out of equilibrium. I will also show how these ideas have been recently implemented in an experiment which enabled us to directly sample the work probability distribution. The “quantum work meter” I will present, built with an ensemble of cold Rubidium atoms manipulated by an atom chip, enabled us to perform a direct test of the most notable fluctuation theorem: the Jarzynski identity. 14:00, CdF, salle 2 05/10/2017 Serge Reynaud (LKB) Métrologie quantique et symétries relativistes La métrologie moderne est basée sur la physique quantique et relativiste. La seconde est définie comme un nombre entier de périodes d’une horloge atomique et le mètre est dérivé de la valeur fixée pour la vitesse de la lumière dans le vide. Bientôt, la constante de Planck et la charge élémentaire devraient être également définies avec des valeurs exactes remplaçant les anciennes définitions du kilogramme et de l’ampère. Le statut quantique des observables associées au temps, à l’espace, à la masse, leur compatibilité avec les symétries relativistes restent pourtant des questions ouvertes. On discutera d’abord de manière qualitative ces questions à l’interface entre métrologie, physique quantique et relativité. On présentera ensuite un cadre théorique où les observables sont définies de manière à être compatibles à la fois avec les exigences relativistes et quantiques. Dans cette approche, la masse observable n’est plus une constante, comme on peut le deviner en raison de sa dimension par rapport à la dilatation. Les transformations des observables vers des référentiels accélérés diffèrent de leurs homologues classiques. Les symétries relativistes permettent néanmoins d’étendre les règles de covariance de la relativité, ce qui conduit à une version quantique du principe d’équivalence d’Einstein identifié à la transformation de la masse observable. 11:00, Danjon (Paris)