# LKB SEMINARS/SÉMINAIRES DU LKB

*Upcoming seminars/Séminaires à venir*

##### 20 mars 2019 - Dieter Meschede (University of Bonn)

ENS – 24, rue Lhomond – 75005 Paris, Room : CONF IV, on wednesday 20 Friday 15th march, at 13H45

**Few Atom Systems on the Route towards Quantum Technology?**

In the so called bottom-up approach to controlled atomic quantum matter small quantum systems are synthesized involving single, few or many atom systems. I will discuss two examples:

I will show that controlled interaction of atoms with a so called fast optical resonators leads not only to a strongly preferred emission of light into optical wave guides (Purcell effect) but also to speed higher than conventional atomic decay rates. This device will be useful for future interconnects in hybrid quantum networks: We have taken first steps towards coupling broad band (e.g. quantum dot) photons to narrow band fiber coupled atomic memories. Quantum networks will have to rely on so called quantum repeaters for large scale distribution of quantum states. Quantum repeaters remain an enormous challenge for experimenters.

With quantum walks – i. e. driven discrete transport on a lattice conditioned on the spin state – we operate a tool realizing controlled coherent transport of atoms over tens of lattice sites – up to the so called quantum speed limit available. I will present the experimental realization of “ideal negative measurements” showing strong violation of the Leggett-Gard inequality. The experiment distinguishes quantitatively the macro realist’s world from the quantum world. A few more examples including e.g. the creation of artificial magnetic fields will be given. The ultimate aim of these experiments is the creation of quantum cellular automata.

##### 27 mars 2019 - Laurent Daudet (LightOn)

**Optical random features for large-scale machine learning**

The propagation of coherent light through a thick layer of scattering material is an extremely complex physical process. However, it remains linear, and under certain conditions, if the incoming beam is spatially modulated to encode some data, the output as measured on a sensor can be modeled as a random projection of the input, i.e. its multiplication by an iid random matrix. One can leverage this principle for compressive imaging, and more generally for any data processing pipeline involving large-scale random projections. This talk will discuss recent technological developments of optical co-processors within the startup LightOn, and present a series of proof of concept experiments in machine learning, such as transfer learning, change point detection, or recommender systems.

##### 03 avril 2019 - Markus Oberthaler (University of Heidelberg)

**Quantum Atom Optics: Ocean & Universality in Quantum Dynamics**

The experimental platform of atoms manipulated by light offers answers to a broad spectrum of open questions. With two explicit and very different examples I will give you a glimpse how broad this spectrum is. I will start with a fundamental question in oceanography: At what time has the deep water in the ocean been in exchange with the atmosphere? Quantum atom optics offers the experimental possibility to detect the very rare Argon 39 atoms one by one and with that allows the dating of water samples as small as ten liters [1]. A very different question in physics is about the existence of universal behavior. Specifically in respect to time dynamics this has only recently been discussed theoretically in the context of the early phase of after a heavy ion collision. Universal meaning, that the evolution does not depend on the initial condition and follows the scaling hypothesis in time and space. I will introduce the concept and present the first observation of this phenomenon in highly controlled ultracold Bose gases [2].

[1] Ar-39 dating with small samples provides new key constraints on ocean ventilation, Nature Comm. 9, 5046 (2018).[2] Observation of universal dynamics in a spinor Bose gas far from equilibrium, Nature 563, 217 (2018).*Previous seminars/Séminaires passés*

Date |
Speaker |
Title and abstract |
Time and location |

15/03/2019 | Allard MoskUtrecht University |
Speckle correlations and Image information in turbid media
Random scattering of light, which takes place in paper, paint and biological tissue is an obstacle to imaging and focusing of light and thus hampers many applications. At the same time scattering is a phenomenon of basic physical interest as it allows the study of fascinating interference effects such as open transport channels, which enable lossless transport of waves through strongly scattering materials. These speckle correlation effects are associated with a relatively broad bandwidth, raising the question whether they are associated with light that has undergone a less-than average number of scattering events. |
13:45, Collège de France – 11, place Marcelin-Berthelot – 75005 Paris, Room : 2 |

12/02/2019 | Thomas UdemMax-Planck Garching |
Precision Spectroscopy of Atomic Hydrogen and the Proton Radius Puzzle
Precise determination of transition frequencies of simple atomic systems are required for a number of fundamental applications such as tests of quantum electrodynamics (QED), the determination of fundamental constants and nuclear charge radii. The sharpest transition in atomic hydrogen occurs between the metastable 2S state and the 1S ground state with a natural line width of only 1.3 Hz. Its transition frequency has been measured with almost 15 digits accuracy using an optical frequency comb and a cesium atomic clock as a reference [1]. A measurement of the Lamb shift in muonic hydrogen is in significant contradiction to the hydrogen data if QED calculations are assumed to be correct [2]. In order to shed light on this discrepancy the transition frequency of one of the broader lines in atomic hydrogen has to be measured with very good accuracy. For this purpose we have employed our previous 1S-2S apparatus as a cold source of laser excited 2S atoms in order to perform spectroscopy on the 2S-4P transitions. With a natural line width of 12.7 MHz, large Doppler effects, quantum interference etc. a good line shape analysis is mandatory to identify the true transition frequency. Our result on this transition yields a value for the proton radius that is compatible with the value obtained from muonic hydrogen with an uncertainty that is comparable to the previous hydrogen world data [3]. Meanwhile Helene Fleurbaey and her team at the Laboratoire Kastler Brossel, Paris have re-measured the 1S-3S transition frequency with a significantly improved accuracy and find the previous “regular hydrogen charge radius” [4]. At our lab we have also been working on this transition with a different method. We hope to be ready to report a result soon. This will provide a unique opportunity to compare two highly accurate measurements obtained at different labs.
C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011) |
13:45, ENS – CONF IV – 24, rue Lhomond – 75005 Paris |

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Speaker |
Title and abstract |
Time and location |

10/12/2018 | Pertti HakonenAalto University |
Josephson effect in suspended single-walled carbon nanotubes
We study suspended, 300-nm-long single-walled carbon nanotubes (SWCNT) contacted using MoRe leads. Good contact transparency of the superconductor-nanotube interface allows for the observation of proximity-induced superconductivity in our SWCNT devices. The magnitude of the switching supercurrent ranges up to 50 nA and can be tuned periodically by gate-induced charge. The gate charge modulates the retrapping current even more strongly, and its magnitude becomes vanishingly small far away from the charge degeneracy point. Under rf irradiation, our SWCNT devices display clear Shapiro steps, the shape of which depend on the rf frequency and power. Under certain conditions the observed steps become hysteretic, indicating small dissipation on the Josephson phase dynamics at the rf frequency. In these SWCNT devices we find mechanical resonances around 1.5 GHz, while the Q factors amount up to 15000 near the charge degeneracy point. Mechanical modes can be observed also in the superconducting regime either by mixing by the current-phase relation or by inducing Shapiro steps resonantly with the mechanical mode, which both reflect the interplay between the Josephson dynamics and the mechanical degrees of freedom. In addition, we find bifurcation of Shapiro steps around the mechanical resonance frequency, both at fundamental and subharmonic resonance drives. The understanding of these phenomena within the framework of current biased RCSJ model will be discussed. |
11:00, Jussieu – Amphi Herpin (Bâtiment Esclangon) |

10/10/2018 | Serge ReynaudLaboratoire Kastler Brossel |
Métrologie quantique et relativité
La métrologie moderne est basée sur la physique quantique et la relativité. La seconde est définie comme un nombre entier de périodes d’une transition atomique et le mètre est déduit de la valeur fixée pour la vitesse de la lumière dans le vide. Très prochainement, la constante de Planck et la charge élémentaire vont être fixées avec des valeurs exactes qui remplaceront les anciennes définitions du kilogramme et de l’ampère. Le nouveau SI va réaliser le programme d’universalité des unités engagé lors de la création du système métrique. Il pose aussi des questions nouvelles à l’interface entre physique quantique, relativité et métrologie. On discutera ces questions et on présentera une esquisse de cadre théorique où des observables sont définies pour le temps, l’espace et la masse de manière à être compatibles avec les exigences relativistes et quantiques. |
13:45, Jussieu – Tour 13/23, salle 210 |

09/10/2018 | Vincent Debierre Max Planck Institute of Heidelberg |
Hydrogen-like systems from low to high-Z: van der Waals interactions and the bound electron g-factor
We present theoretical results on hydrogen atoms and hydrogen-like ions. |
13:45, Jussieu – Tour 13/23, salle 210 |

26/09/2018 | Holger MüllerUC Berkeley Physics |
Measurement of the fine structure constant as test of the standard model
Measurements of the fine-structure constant are powerful tests of the consistency of theory and experiment in physics. Using the recoil frequency of cesium-133 atoms in a matter-wave interferometer, we recorded a measurement of the fine-structure constant α = 1/137.035999046(27) at 0.20 parts per billion accuracy using multiphoton interactions such as Bragg diffraction and Bloch oscillations. Comparison with Penning trap measurements of the electron gyromagnetic anomaly via the Standard Model of particle physics has implications for dark-sector candidates and electron substructure. We will close with an outlook on future applications of matterwave interferometry. |
13:45, Jussieu – Tour 13/23, salle 210 |

21/09/2018 | Edward HindsImperial College London |
Magneto-optical trapping and sub-Doppler cooling of molecules
Atomic physics has been revolutionised by the introduction of laser techniques to cool atoms far below the Doppler limit. Now, it has become possible to laser cool molecules, to collect them in a magneto-optical trap, to cool them below the Doppler limit [1,2] and to trap them with modest magnetic fields [3]. These ultracold molecules open up a wide vista of future applications. To give a few examples, they can be optically or magnetically trapped to form arrays for quantum simulation, they can make a molecular fountain for testing fundamental physics at unprecedented levels of sensitivity, and they open a new energy range for the study of ultracold collisions and ultracold chemistry. I will review the current status of this field. |
13:45, Collège de France- Salle 5 |

19/09/2018 | Nir DavidsonWeizmann Institute of Science Israel |
Opto-mechanical strain and anomalous dynamics with trapped atoms
We report the observation of the opto-mechanical strain applied to ultra-cold rubidium atoms when shined on by an intense, far detuned homogenous laser beam. This force acts perpendicular to the laser beams direction and depends on the atomic cloud density profile, effectively generating interactions between the atoms. We refer to this force as electrostriction, since it resembles shape changes of materials under the application of a static electric field. We experimentally demonstrate the basic features of electrostriction, distinguishing it from the well-established scattering and dipole forces, and proving it is a new type of force [1]. We also report on experimental study the anomalous real-space and phase-space dynamics of ultra-cold atoms in a one dimension where the spatial distribution exhibits fractional self-similarity and Lévy statistics [2]. We show that the position-velocity correlation function builds up on a timescale related to the initial conditions of the ensemble and then decays asymptotically as a power-law, following a simple scaling theory involving the power-law asymptotic dynamics of position and velocity [3]. Finally, for trapped atoms we demonstrate a breakdown of equipartition theorem [4] predicted for anomalous dynamics in [5]. 1. N. Matzliah, H. Edri, A. Sinay, R. Ozeri, and N. Davidson, Phys. Rev. Lett. 119, 163201 (2017). 2. Y. Sagi, M. Brook, I. Almog, and N. Davidson, Phys. Rev. Lett. 108, 093002 (2012). 3. G. Afek, J. Coslovsky, A. Courvoisier, O. Livneh, and N. Davidson, Phys. Rev. Lett. 119, 060602 (2017). 4. G. Afek, A. Courvoisier, A. Cheplev, and N. Davidson, submitted. 5. Dechant, D. Kessler, and E. Barkai, Phys. Rev. Lett. 115, 173006 (2015). |
13:45, Collège de France – Bât. A, RDC – Salle 2 |

12/09/2018 | Patrick DupréLaboratoire de Physico-Chimie de l’Atmosphère |
Frequency Modulation Spectroscopy in Cavity, Challenging the Precision and the Sensitivity: Application to HD
The contemporary spectroscopy of gas targets several objectives, such as the fine description of the molecular Hamiltonians, the trace detection of diverse species, and the fundamental physics. In simple molecules such as molecular hydrogen, the transitions between energy levels can be probed to test the Standard Model of the Physics (i.e., the Quantum ElectroDynamics), and beyond. For example, the proton-to-electron mass ratio and the proton-radius size can be challenged. On other side, the detection of trace of molecules is of crucial importance for understanding chemical processes and molecular line shape. Up-to-date spectroscopy techniques based on optical cavities associated with Optical Frequency Combs have been developed in the Near-Infrared range to challenge sensitivity and frequency precision. We will illustrate the capability of the Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS), from both theoretical and experimental approaches. Very recent results on a few molecular species (C2H2 and HD) will be discussed for questioning the ultimate resolution and sensitivity which can be targeted. |
13:45, Jussieu tour 13-23 2ème étage. |

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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. |
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 electronsQuantum 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 MICROSCOPELes 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 theoremsThe 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) |

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21/06/2017 | Raphael Lopes( Cambridge) |
On strongly interacting homogeneous Bose—Einstein condensatesDuring 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 gasesTransport 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 SystemPeriodically 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 matterMicro- 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. |
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. |
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 |