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We report the preparation and observation of single atoms of dysprosium in arrays of optical tweezers with a wavelength of 532 nm imaged on the intercombination line at 626 nm. We use the anisotropic light shift specific to lanthanides and in particular a large difference in tensor and vector polarizabilities between the ground and excited states to tune the differential light shift and produce tweezers in near-magic or magic polarization. This allows us to find a regime where single atoms can be produced and imaged. Using the tweezer array toolbox to manipulate lanthanides will open new research directions for quantum physics studies by taking advantage of their rich spectrum, large spin and magnetic dipole moment.

Driven by the need to develop efficient atom-photon interfaces, recent efforts have proposed replacing cavities by large arrays of cold atoms that can support subradiant or superradiant collective states. In practice, subradiant states are decoupled from radiation, which constitutes a hurdle to most applications. In this work, we study theoretically a protocol that bypasses this limit using a one dimensional (1D) chain composed of N three-level atoms in a V-shaped configuration. Throughout the protocol, the chain behaves as a time-varying metamaterial: enabling absorption, storage and on-demand emission in a spectrally and spatially controlled mode. Taking into account the quantum nature of atoms, we establish the boundary between the linear regime and the nonlinear regime. In the nonlinear regime, we demonstrate that doubly-excited states can be coherently transferred from superradiant to subradiant states, opening the way to the optical characterization of their entanglement., Comment: 13 pages, 10 figures

A class of systems exists in which dissipation, external drive and interactions compete and give rise to non equilibrium phases that would not exist without the drive. There, phase transitions could occur without the breaking of any symmetry, yet with a local order parameter, in contrast with the Landau theory of phase transitions at equilibrium. One of the simplest driven dissipative quantum systems consists of two-level atoms enclosed in a volume smaller than the wavelength of the atomic transition cubed, driven by a light field. The competition between collective coupling of the atoms to the driving field and their cooperative decay should lead to a transition between a phase where all the atomic dipoles are phaselocked and a phase governed by superradiant spontaneous emission. Here, we realize this model using a pencil-shaped cloud of laser cooled atoms in free space, optically excited along its main axis, and observe the predicted phases. Our demonstration is promising in view of obtaining free-space superradiant lasers or to observe new types of time crystals., 9 pages, 8 figures

We report the efficient and fast ($\sim 2\mathrm{Hz}$) preparation of randomly loaded 1D chains of individual $^{87}$Rb atoms and of dense atomic clouds trapped in optical tweezers using a new experimental platform. This platform is designed for the study of both structured and disordered atomic systems in free space. It is composed of two high-resolution optical systems perpendicular to each other, enhancing observation and manipulation capabilities. The setup includes a dynamically controllable telescope, which we use to vary the tweezer beam waist. A D1 $\Lambda$-enhanced gray molasses enhances the loading of the traps from a magneto-optical trap. Using these tools, we prepare chains of up to $\sim 100$ atoms separated by $\sim 1 \mathrm{\mu m}$ by retro-reflecting the tweezer light, hence producing a 1D optical lattice with strong transverse confinement. Dense atomic clouds with peak densities up to $n_0 = 10^{15}\:\mathrm{at}/\mathrm{cm}^3$ are obtained by compression of an initial cloud. This high density results into interatomic distances smaller than $\lambda/(2\pi)$ for the D2 optical transitions, making it ideal to study light-induced interactions in dense samples.

Manifestations of Quantum Physics at the thermodynamical level are found in a broadrange of physical systems. A famous example is superfluidity, discovered at the beginningof the 20th century and found in many different situations, from liquid helium to neutronstars. Dilute ultracold gases offer a unique versatility to engineer quantum many-bodysystems, which can be directly compared with theory thanks to the controllability of theirenvironment. In this thesis we present several experimental investigations led on ultracoldlithium gases. Lithium provides the possibility to study ensembles of bosons andfermions, with controllable interactions between the constituents. We present experimentaltechniques for preparation and studies of degenerate gases of lithium, with prospects forimprovement of the existing methods. We first report on an investigation of three-bodyrecombination of bosons under a resonant two-body interaction. This study, quantitativelycompared with theory constitutes a benchmark for further studies of the unitary Bose gas.Finally, we present the first experimental realization of a mixture of a Bose superfluid witha Fermi superfluid. We demon- strate that both components are in the superfluid regime,and that the counter-flow motion between them possesses the characteristics of superfluidflow, with the absence of viscosity below a critical velocity, and an onset of friction above.Using collective oscillations of the mixture, we measure the coupling between the twosuperfluids in close agreement with a theoretical model., On trouve des manifestations de la physique quantique au niveau thermodynamique dansde nombreux systèmes. Un exemple marquant est la superfluidité, découverte au début du20ème siècle, que l’on retrouve de l’hélium aux étoiles à neutrons. Les gaz dilués ultrafroidsoffrent une polyvalence unique pour étudier des systèmes quantiquesmacroscopiques, pouvant directement tester les théories grâce à un environnementcontrôlé. Dans cette thèse, nous présentons plusieurs études expérimentales de gaz froidsde lithium. Le lithium fournit la possibilité de réaliser des ensembles de bosons et defermions, avec des interactions contrôlables entre les constituants. Nous présentons lestechniques utilisées pour préparer et étudier des gaz dégénérés de lithium, et uneamélioration possible des méthodes existantes. Nous décrivons premièrement une étudede la recombinaison à trois bosons avec une interaction à deux corps résonante. Comparésquantitativement à la théorie, ces résultats fournissent une référence pour les étudesfutures du gaz de Bose unitaire. Pour finir, nous présentons la première observationexpérimentale d’un mélange de superfluides de Bose et de Fermi. Nous démontrons queles deux composants sont superfluides et que leur écoulement relatif vérifie les propriétésdes écoulement superfluides, avec une absence de viscosité en dessous d’une vitessecritique puis la présence de dissipation au-delà. En utilisant des excitations collectives dece mélange, nous mesurons l’interaction entre les deux superfluides, en accord avec unmodèle théorique.

We determine {\`a} la Landau the critical velocity $v\_c^{L}$ of a moving impurity in a Fermi superfluid, that is by restricting to the minimal excitation processes of the superfluid. $v\_c^{L}$ is then the minimal velocity at which these processes are energetically allowed. The Fermi superfluid actually exhibits two excitation branches~: one is the fermionic pair-breaking excitation, as predicted by BCS theory; the other one is bosonic and sets pairs into motion, as predicted by Anderson's RPA. $v\_c^{L}$ is the smallest of the two corresponding critical velocities $v\_{c,f}^{L}$ and $v\_{c,b}^{L}$. In the parameter space (superfluid interaction strength, fermion-to-impurity mass ratio), we identify two transition lines, corresponding to a discontinuity of the first-order and second-order derivatives of $v\_c^{L}$. These two lines meet in a triple point and split the plane in three domains. We briefly extend this analysis to the very recently realized case at ENS, where the moving object in the Fermi superfluid is a weakly interacting Bose superfluid of impurities, rather than a single impurity. For a Bose chemical potential much smaller than the Fermi energy, the topology of the transition lines is unaffected; a key result is that the domain $v\_c^{L}=c$, where $c$ is the sound velocity in the Fermi superfluid, is turned into a domain $v\_c^{L}=c+c\_B$, where $c\_B$ is the sound velocity in the Bose superfluid, with slightly shifted boundaries., Comment: final version, both in english and in french

Following the bichromatic sub-Doppler cooling scheme on the D1 line of 40K recently demonstrated in (Fernandes et al. 2012), we introduce a similar technique for 7Li atoms and obtain temperatures of 60 uK while capturing all of the 5x10^8 atoms present from the previous stage. We investigate the influence of the detuning between the the two cooling frequencies and observe a threefold decrease of the temperature when the Raman condition is fulfilled. We interpret this effect as arising from extra cooling due to long-lived coherences between hyperfine states. Solving the optical Bloch equations for a simplified, \Lambda-type three-level system we identify the presence of an efficient cooling force near the Raman condition. After transfer into a quadrupole magnetic trap, we measure a phase space density of ~10^-5. This laser cooling offers a promising route for fast evaporation of lithium atoms to quantum degeneracy in optical or magnetic traps., Comment: 8 pages, 8 figures; submitted to Phys. Rev. A; V2 contains edits in response to referee's comments