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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 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)
 Date Speaker Title and abstract Time and location 21/06/2017 Raphael Lopes (Cambridge) On strongly interacting homogeneous Bose—Einstein condensates During this talk, I will show that using two-photon Bragg spectroscopy, one can study the energy of particle-like excitations in a strongly interacting homogeneous Bose-Einstein condensate, and observe dramatic deviations from Bogoliubov theory. In particular, at large scattering length a the shift of the excitation resonance from the free-particle energy changes sign from positive to negative. For an excitation with wavenumber q, this sign change occurs at a about 4/(\pi q), in agreement with the Feynman energy relation and the static structure factor expressed in terms of the two-body contact. For a about 3/q we also see a breakdown of this theory, and better agreement with calculations based on the Wilson operator product expansion. Neither theory explains our observations across all interaction regimes, inviting further theoretical efforts.Moreover, we also show that through the use of a Bragg filtering method one can access the momentum distribution of an ultra-cold atomic gas and extract its condensed fraction. We observe that the condensed fraction reduces linearly as a function of (na^3)^0.5, behaviour which follows the quantitative prediction of quantum depletion introduced by N. N. Bogoliubov 70 years ago. 13:45, CdF, salle 5 15/03/2017 Joseph H. Thywissen (University of Toronto) Spin transport in 3D and 2D strongly interacting Fermi gases Transport in strongly interacting systems is a topic of broad interest to physicists, studied in materials, fluids, cold atoms, and even in theories using holographic duality. Here we study spin transport in the demagnetization dynamics of a strongly interacting ultracold Fermi gas. Atoms are initialized in a superposition of two internal states, creating a transverse magnetization that decays in the presence of a magnetic field gradient. We observe the ensemble-averaged magnetization with a spin-echo sequence, and measure two-body correlations (the contact) with time-resolved rf spectroscopy.In the strongly interacting regime, the dynamics are found to be diffusive. The spin diffusivity reaches a lower bound, roughly 2 hbar/m (where m is the bare mass of the potassium 40 atoms used here), when interactions are tuned to unitarity. We also find a reactive component to dynamics, due to the spin-rotation effect, where the spin current precesses around the local magnetization. Finally, we compare dynamics in three- and two-dimensional gases. Our work supports the conjecture of an upper bound on the rate of relaxation to local equilibrium. 13:45, ENS, salle Conf IV 15/02/2017 Wonho Jhe (Seoul National University) Nonlinear, Nonequilibrium, Collective Dynamics in a Periodically Modulated Cold Atom System Periodically modulated systems form one of the most important classes of nonequilibrium systems, both conceptually and in terms of applications. They have discrete time-translation symmetry: they are invariant with respect to time translation by modulation period tF. Nevertheless, they have stable vibrational states with periods of 2tF, that is period doubling. Interestingly, in a many-body system, dynamical period doubling in itself does not break the time translation symmetry, a consequence of fluctuations. However, if as a result of the interaction the state populations become different, the symmetry is broken, an Ising-class phase transition. We show that an atomic system in a periodically modulated optical trap displays an ideal mean-field symmetry-breaking transition, which is a critical phenomenon as demonstrated by experimental measurement of critical exponents. We also discuss the dynamic phase transition as well as kinetic phase transition observed in the modulated atom trap. 13:45, CdF, salle 2 01/02/2017 Jürgen Volz (VCQ, Vienne) Quantum optical nonreciprocal devices based on chiral interaction of light and matter Micro- and nanophotonic components such as optical nanofibers confine light at the wavelength scale and enable the control of its flow in integrated optical environments. The strong confinement of light leads to an inherent link between its local polarization and propagation direction – the light obtains a chiral character. This fundamentally alters the physics of light-matter interaction and gives rise to phenomena such as highly directional spontaneous emission and direction-dependent coupling strengths [1].In my talk I will introduce the underlying principles of this chiral light-matter interaction. Then I will present how we employ this effect to realize low-loss nonreciprocal transmission of light at the single-photon level through a silica nanofiber [2]. We use two experimental approaches where an ensemble of spin-polarized atoms is weakly coupled to the nanofiber-guided mode or a single spin-polarized atom is strongly coupled to the nanofiber via a whispering-gallery-mode resonator. We observe a strong imbalance between the transmissions in forward and reverse direction of about 10 dB for both systems while, at the same time, the forward transmissions still exceed 70%.By interfacing a whispering-gallery-mode resonator with two nanofibers we extended this system to a 4-port device, where a single atom routes photons nonreciprocally from one fiber port to the next, thus realizing a quantum optical circulator [3]. The demonstrated systems exemplify a new class of nanophotonic devices based on chiral light-matter interaction. Since their operation direction can be controlled by individual quantum emitters, such devices could even be prepared in a superposition of their operational modes which allows their integration in future quantum information processing and quantum simulation experiments.[1] P. Lodahl et al., arXiv:1608.00446 (2016)[2] C. Sayrin et al., Phys. Rev. X 5, 041036 (2015)3] M. Scheucher et al., Science 354, 1577 (2016) 13:45, CdF, salle 2 25/01/2017 Nicolas Joly (MPI, Erlangen) Génération de sources non classiques au moyen de fibres microstructurées Les fibres microstructurées sont un excellent outil pour l’optique non linéaire en raison de la possibilité de contrôler et modeler leurs propriétés de dispersion et leur non linéarité. Dans ce séminaire je présenterai différentes fibres dédiées à la génération d’états non classiques de la lumière. Dans un premier temps je montrerai la création de faisceaux jumeaux corrélés, créés par instabilité modulationelle. Le système est particulièrement simple et versatile et se compose d’une fibre à cœur creux remplie d’argon gazeux pompée par un laser saphir-titane. L’utilisation de gaz comme élément non linéaire permet de modifier à souhait la dispersion du système qui, une fois ajustée correctement, permet à des impulsions (300 fs) de développer des bandes latérales corrélées. De plus la source est spatialement monomode et présente un petit nombre de modes temporels (<5). Dans un deuxième temps, je présenterai un nouveau concept de fibre microstructurée permettant, en principe, la création d’état triplet. La génération d’état triplet, dans laquelle un photon donne spontanément naissance à trois photons, peut être considérée comme le processus inverse du triplement de fréquence et nécessite par conséquent les mêmes conditions d’accord de phase. La dispersion chromatique impose cependant l’utilisation de différents modes spatiaux, ce qui est techniquement délicat. Nous avons récemment proposé un concept de fibre présentant deux modes de guidance différents : aux grandes longueurs d’onde la guidance s’opère par réflexion totale alors que la troisième harmonique est guidée par effet de bande interdite. Les premières expériences montrent la possibilité d’un accord de phase entre deux modes fondamentaux. 13:45, ENS, L357/L356 18/01/2017 Nouveaux doctorants et postdocs (LKB) Présentations des nouveaux arrivants du LKB Jour II 13:45, CdF, salle 5 14/12/2016 Nouveaux doctorants et postdocs (LKB) Présentations des nouveaux arrivants du LKB Jour I 11:00, CdF, salle 2 30/11/2016 Franklyn Quinlan (NIST, USA) Optical to RF frequency generation with optical frequency combs The most frequency-stable electromagnetic radiation is now produced optically, with stable reference cavities demonstrating fractional frequency instabilities below 10^-16 at 1 second and optical clocks reaching 10^-18 at 10^4 seconds. This talk will cover recent work at NIST using optical frequency combs to transfer this stability across the optical domain at the level of 10^-18 at 1 second, as well as into the RF, microwave, and mm-wave domains at the level of 10^-15 to 10^-17 at 1 second. In addition to the optical frequency combs themselves, elements of compact ultrastable optical cavities, high-speed photodetection and broadband electronic synthesis will also be discussed. 13:45, CdF, salle 2 23/11/2016 Boris Svistunov (UMass Amherst, USA) Superfluidity and Topological Order The decades-long history of theoretical understanding of the phenomenon of superfludity—from the Tisza-Landau phenomenology to the modern picture based on the emergence of topological constant of motion—has been most dramatic and instructive. Critically overviewing main historic steps highlights the crucial importance of the topological language for revealing the origin the phenomenon, as well as for establishing general relations of superfluid hydrodynamics, Berezinskii-Kosterlitz-Thouless transition, and superfluid-insulator quantum phase transitions in one dimension. 13:45, CdF, salle 5 9/11/2016 Emmanuel Fort (Institut Langevin) Self-propelled droplets: A ‘classical’ wave-particle duality based on time mirrors We have recently discovered a macroscopic object composed of a material particle dynamically coupled to a wave packet. The particle is a droplet bouncing on the surface of a vertically vibrated liquid bath; its pilot-wave is the result of the superposition of the surface waves it excites. Above an excitation threshold, this symbiotic object, designated as a “walker” becomes self-propelled. Such a walker exhibits several features previously thought to be specific to the microscopic realm. The unexpected appearance of both uncertainty and quantization behaviors at the macroscopic scale lies in the essence of its “classical” duality. The dynamics of the droplet depends on previously visited spots along its trajectory through the surface waves emitted during each bounce. This path memory dynamics gives a walker an intrinsic spatio-temporal non-locality. I will discuss the characteristics of these objects that encode a wave memory. In particular, I will introduce the concept of temporal mirrors and time crystals to interpret the characteristics of the driving wave packet. 13:45, CdF, salle 2 26/10/2016 Giacomo Roati (University of Florence) Dynamics of strongly interacting atomic Fermi gases I will present two antithetic experimental studies, exploiting strongly interacting ultracold Fermi gases of 6Li atoms confined in optical potentials. In a first experiment, we create the analogous of a Josephson junction by bisecting BEC-BCS crossover superfluids by a thin optical barrier. We observe coherent dynamics in both the population and in the relative phase between the two superfluid reservoirs. For critical parameters, we see how the Josephson dynamics is affected by the presence of topological defects entering the superfluid bulk [1]. In a second experiment, we create an artificial ferromagnetic state by segregating degenerate spin mixtures into two initially disconnected reservoirs [2]. We study the spin dynamics for different interaction strengths and temperatures. For sufficiently high values of the inter-spin repulsive interactions and sufficiently low temperatures, we observe a softening of the spin dipole mode and a time-window during which spin diffusion is zeroed. Our measurements provide exciting new insights into the physics of attractive and repulsive Fermi gases. [1] G. Valtolina et al., Science 350, 1505 (2015)[2] G. Valtolina et al., arXiv:1605.07850v1 (2015) 13:45, CdF, salle 5 19/10/2016 Sebastian Will (Columbia University) Coherent Quantum Control of Ultracold Dipolar NaK Molecules Ultracold molecules open up new routes for precision measurements, quantum information processing and many-body quantum physics. In particular, dipolar molecules with long-range interactions promise the creation of novel states of matter, such as topological superfluids and quantum crystals. Dipolar bialkali molecules can be efficiently assembled from ultracold atoms. Using this approach we have created the first near-degenerate gases of strongly dipolar NaK molecules. At temperatures of few hundred nanokelvin, we prepare ensembles, in which all molecules occupy the rovibrational and hyperfine ground state. In my talk, I will discuss our recent progress on coherent quantum control of trapped, ultracold NaK molecules. Starting from the absolute ground state, we demonstrate microwave transfer into excited rotational and hyperfine states, and develop a thorough understanding of NaK’s rich hyperfine structure in the presence of static magnetic and electric fields. Building on this analysis, we show coherent two-photon microwave coupling between the two lowest nuclear spin states of NaK. For superpositions of these states, we observe coherence times of up to one second, enabling Ramsey spectroscopy with Hertz-level resolution. 13:45, CdF, salle 2
 Date Speaker Title and abstract Time and location 20/07/2016 Guillaume Pignol (LPSC, Grenoble) La symétrie T à l’épreuve des neutrons ultrafroids L’asymétrie entre la matière et l’antimatière observée dans l’Univers reste inexpliquée. Une physique microscopique nouvelle, au delà du Modèle Standard, était sûrement à l’œuvre dans cette phase hypothétique de l’Univers primordiale qu’on appelle la baryogenèse. Cette nouvelle physique, violant la symétrie par renversement du temps T, pourrait être révélée en laboratoire en mesurant le moment dipolaire électrique du neutron (nEDM). En effet, l’existence d’un nEDM non nul n’est pas compatible avec la symétrie T. L’appareil qui a produit la meilleure limite en 2006 (le nEDM est encore compatible avec zéro, pour l’instant…) a fait peau neuve et produit actuellement des données auprès de la source de neutrons ultrafroids de PSI en Suisse. Je présenterai cette expérience en insistant sur le point central: le contrôle du champ magnétique. Notamment, nous utilisons un magnétomètre atomique 199Hg. Récemment, nous avons pu comparer le moment magnétique du neutron et du mercure avec une précision meilleure que le ppm, revisitant la mesure de Bernard Cagnac au LKB en 1960 ! 13:45, Jussieu, salle conf 06/07/2016 Olivier Pfister (Uni Virginia) Engineering noninteracting-boson fields: from squeezed measurement noise to large-scale entanglement for quantum computing Quantum optics may be viewed as the optics of manifestly nonclassical waves, such as matter waves, and it may also be understood as the nonclassical optics of any wave. The latter definition is more universal as it involves the quantum effect of particle statistics on wave phenomena such as interference, with a central role played by vacuum field modes. In this seminar, I will present several quantum optics experiments in which photon statistics are altered, or squeezed, away from the vacuum noise, for 1 to 60 modes of the same optical parametric oscillator. This multimode squeezing can be directly applied to secure communication, high-precision interferometry, and to generate continuous-variable cluster states for a possible record size (if not yet record fidelity) quantum computer. 13:45, CDF, salle 5 22/07/2016 Zoran Hadzibabic (Uni Cambridge) Quantum Gas in a Box For the past two decades harmonically trapped ultracold atomic gases have been used with great success to study fundamental many-body physics in a flexible experimental setting. Recently, we have achieved the first atomic Bose-Einstein condensate in an essentially uniform potential of an optical-box trap [1], which has opened new research possibilities, such as for quantitative studies of critical phenomena near phase transitions [2]. In this seminar I will present some of our most recent experiments on this new system, including the study of turbulence in a continuously driven gas. [1] A. L. Gaunt, T. F. Schmidutz, I. Gotlibovych, R. P. Smith, and Z. Hadzibabic, “Bose-Einstein Condensation of Atoms in a Uniform Potential”, Phys. Rev. Lett. 110, 200406 (2013)[2] N. Navon, A. L. Gaunt, R. P. Smith, and Z. Hadzibabic, “Critical Dynamics of Spontaneous Symmetry Breaking in a Homogeneous Bose Gas”, Science 347, 167 (2015) 13:45, CDF, salle 5 20/04/2016 Leonardo Mazza (ENS) Majorana fermions: a cold-atom perspective Is it possible to observe zero-energy Majorana modes in a cold-atom experiment? How should we modify the established theory for solid-state materials to account for the typical properties of these setups? In this talk I will discuss the problem of observing Majorana fermions in a closed setup where the number of particles is conserved. I will present an exactly-solvable and easy-to-handle model which is general enough to share most of the expected features of Majorana fermions in number-conserving systems [1]. The model owes a natural extension to a dissipative situation, where a properly tuned environment drives the system into a number-conserving topological phase [2]. The analysis of both models exploits exact results and numerical simulations with matrix product states. Finally, I will conclude with a few remarks on the problem of storing quantum information into these modes [3,4]. [1] Iemini, Mazza, Rossini, Fazio, Diehl, Phys. Rev. Lett. 115, 156402 (2015)[2] Iemini, Rossini, Fazio, Diehl, Mazza, Phys. Rev. B 93, 115113 (2016)[3] Ippoliti, Rizzi, Giovannetti, Mazza, arXiv:1511.06592 (2015)[4] Mazza, Rizzi, Lukin, Cirac, Phys. Rev. B 88, 205142 (2013) 13:45, ENS, salle 235B 06/04/2016 Rafael Piestun (Uni Colorado) Beyond Super-resolution Localization Microscopy: Imaging in three dimensions, multiple colors, dense scenes, with compressive acquisition, and drift mitigation Abbe’s resolution limit has been overcome after more than 130 years enabling unprecedented opportunities for optical imaging at the nanoscale. Fluorescence imaging using photoactivatable or photoswitchable molecules within computational optical systems offers single molecule sensitivity within a wide field of view. In localization microscopy, super-resolution images are generated in a pointillistic fashion by identifying individual fluorescent molecules from thousands of images, where each frame contains only a sparse set of active emitters. Initial demonstrations were limited to two dimensions, required long acquisition times, and relied on post-processing for drift correction. Furthermore, the samples under investigation were thin and contained sparse objects. For localization microscopy techniques to reach their full potential, these challenges had to be addressed. The advent of three-dimensional point spread function engineering associated with optimal reconstruction algorithms provides a unique approach to further increase resolution in three dimensions. The work presented in this talk summarizes efforts to expand the capabilities of localization microscopy in order to broaden the impact in live-cell biological research and enable further scientific discoveries. 13:45, ENS, salle 235B 23/03/2016 David McClelland (Australian National University) GW detection and squeezing In February 2016, the LIGO and Virgo Collaborations announced the detection of gravitational waves and with it the beginning of the new field of gravitational wave astronomy. As with all fields of astronomy, the quest to understand the universe will demand detectors with better and better sensitivity. Future interferometric detectors of gravitational waves are predicted to be primarily limited by quantum noise. Non-classical states of light will thus feature in driving the sensitivity of such detectors to sense the universe out to cosmological distances. In this talk, after briefly introducing gravitational waves and GW astronomy, I will review possible/proposed detector configurations, examine requirements on squeezed light sources imposed by these configurations and summarise where we are at currently. 13:45, ENS, salle 235C 09/03/2016 Mauro Paternostro (Uni Belfast) Irreversibility in non-equilibrium quantum processes: a mesoscopic physics study I will discuss the implication of non-equilibrium quantum thermodynamics for logical irreversibility. I will assess this issue by discussing the role played by the irreversible thermodynamic entropy produced by driving a quantum system out of equilibrium. Such framework will be then illustrated in the context of two recent experiments, one in cavity optomechanics and one involving intra-cavity ultra-cold atom systems, which demonstrate the experimental accessibility of non equilibrium thermodynamics in mesoscopic quantum systems. 13:45, ENS, salle 235C 27/01/2016 Max Lesaffre et Bertrand de Fürst (SATT Lutech) Transférez vos résultats de Recherche Seront abordés les enjeux, les processus et les outils et acteurs de la Valorisation de la Recherche. Comment protéger ses résultats ? Qu’est-ce que la maturation technologique ? Quel processus suit le transfert ? Les enjeux de la création de start-up. Comment être accompagné ? Ces différents points permettront aux personnels des laboratoires d’avoir une vision globale sur ces notions de transfert, et d’être sensibilisés aux enjeux et outils impliqués. 13:45, ENS, conf IV 13/01/2016 Paulo Maia Neto (IF-UFRJ, Rio de Janeiro) Towards Casimir force measurements with optical tweezers We propose to use optical tweezers to probe the Casimir interaction between microspheres at distances far beyond the validity of the widely employed proximity force approximation. To demonstrate the feasibility of our proposal, we have measured double layer forces in the range ~10 fN by employing ultra-soft optical tweezers. 13:45, ENS, conf IV 16/12/2016 Séminaire des nouveaux doctorants 13:45, CDF, salle 5 9/12/2015 Thomas Wellens (Uni Freiburg) Scattering laser light on cold atoms: Multiple scattering signals from single-atom responses The theory of multiple scattering in dilute media that consist of a disordered collection of discrete scatterers relies on the division of the total scattering process into single scattering events. In standard multiple scattering theory, these are assumed to be linear (scattered field proportional to incident field). For atomic scatterers with transition frequency close to the laser frequency, however, nonlinear multi-photon scattering processes are induced at high laser intensities. To account for the impact of these processes on the multiple scattering signal, we present an approach which combines tools of diagrammatic multiple scattering theory (ladder and crossed diagrams) with quantum-optical methods (optical Bloch equations). This approach allows us to evaluate how quantum-mechanical scattering processes influence, both, diffusive propagation of the average light intensity through a dilute cloud of cold atoms (with distances between the atoms much larger than the laser wavelength), as well as effects of coherent light propagation such as coherent backscattering. 13:45, Jussieu, salle conf 21/10/2015 Sanli Faez (Uni Utrecht) Optical tracing of single conduction electrons via organic molecules I introduce an optical platform for optical tracing of quasi-static charge distributions, down to the elementary charge level using single organic molecules. Scanning probe microscopy, which has been instrumental to quantum nanoscience, is inherently limited to measurements on one single spot at a time. An enormous advantage of optical methods is their capability of measuring positions of multiple electrons simultaneously. In our approach single molecules with lifetime-limited optical linewidths at cryogenic temperatures are used as tiny local field probes that provide nanometer resolution and high electric field sensitivity. As the first essential step towards the realization of this device, I present experimental demonstration of efficient coherent interaction between propagating photons of a light beam and a single molecule. Our system consists of a glass nanocapillary filled with an organic crystal doped with dye molecules. The filled capillary core with a diameter of 600 nm guides a confined mode for the excitation light and provides a spontaneous emission coupling factor of up to beta = 0.18. I present extinction spectra recorded in transmission and sharp bright resonance fluorescence spectra from the side. This arrangement paves the way for controlled experiments on coherent optical cooperative effects involving small ensembles of organic molecules. By exploiting the excellent quantum coherence of single organic emitters, this platform also sets the scene for a solid-state hybrid quantum interface between a stationary superconducting qubit and a photonic flying qubit that can be transmitted over long distances. 11:30, Jussieu, salle conf 7/10/2015 Jerome Esteve (Uni Paris Sud) Le projet Topolux : vers des états topologiques de la lumière dans le régime micro-onde De nombreuses propositions théoriques existent pour réaliser des états quantiques fortement corrélés dans des systèmes photoniques (“quantum fluids of light”), et en particulier des états topologiques comme, par exemple, ceux de l’effet Hall quantique fractionnaire. Dans cet exposé, nous présenterons le projet Topolux récemment lancé au LPS dont l’objectif à long terme est d’implémenter ces idées en réalisant des réseaux 2D pour photons micro-onde avec un champ de jauge artificiel et un terme d’interaction. Ces réseaux seront fabriqués en assemblant les briques élémentaires développées ces dernières années en “circuit QED” : résonateurs, qubits supraconducteurs… Nous essaierons également de présenter l’état de l’art des expériences dans ce domaine émergeant. 13:45, Jussieu, salle conf