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Atom chip
 We study the physics of onedimensional Bose gases using an atom chip setup. Rubidium 87 atoms are held in a magnetic surface trap allowing strong transverse confinement. The variety of phases in this reduced dimension is very rich, from the weakly interacting quasicondensates to the fermionised regimes and strongly correlated phases, and the abundant theoretical tools allow quantitative comparison between theory and experiment. Moreoever the uniform 1D Bose gas has an integrable hamiltonian making this system an ideal test bench for studying out of equilibrium dynamics in the very active context of relaxation of isolated quantum manybody systems. 


Discribing the outofequilibrium dynamics of manybody quantum systems is a priori a tremendously difficult task. However, a recent theoretical development provides an abinitio description of the long wavelength dynamics of 1D integrable quantum systems, the socalled Generalised Hydrodynamics (GHD). In contrast to conventional hydrodynamics, GHD does not assume that gas is locally described by the Gibbs ensemble but it keeps track of all conserved quantities of the integrable system. In cold atom experiments, 1D bosonic gases are realised, which are well described by the famous integrable LiebLiniger model. Cold atom experiments thus offer an ideal platform to test GHD.
We use our atomchip experiment to test experimentally GHD. Starting from a cold atomic cloud at thermal equilibrium, dynamics is generated by a sudden quench of the longitudinal potential. The measured time evolution of the density profiles are in excellent agreement with predictions from GHD. We also compare our data with predictions from the conventional hydrodynamics method, which assumes locally a thermal equilibrium described by a Gibbs ensemble. Except for the special case of harmonic potentials, we find that conventional hydrodynamics completely fails to reproduce our data. Hydrodynamics even predicts the development of sharp structures leading to a chock phenomena, such a phenomena being absent in the data and in the GHD description.
Cold atomic cloud are metastable states, the ground state being a solid, and the cold clouds are plagged with 3body process were a depply bound molecule is formed in the course of a 3body collision. The binding energy is released in the form of kinetic energy of the molecule and the remaining atom, typically much larger than the trap depth and the 3body process thus amount to the loss of 3 atoms : each recombination event results in the loss of 3 atoms. As a dissipative process, the 3body losses are usually considered as detrimental for cold gases. Since they occur predominantly in the regions of small trapping energy, where the atomic density is the largest, they are known to heat a thermal gases by an antievaporation process. In our experiment, we demonstrate experimentally for the first time cooling due to 3body losses. More precisely, we use a harmonically confined onedimensional Bose gas in the quasicondensate regime and use density ripples analysis to extract the temperature of the phonon modes. As the atom number decreases under the effect of threebody losses, the temperature drops up to a factor four. The ratio $k_B T /mc^2$ stays close to 0.64, where $c$ is the speed of sound in the trap center and $m$ the atomic mass. This value is close to the stationnary value predicted by the theory. In terms of the 1D dimensionless parameter $\gamma$ characterizing the strength of the interactions, our different data sets span more than two orders of magnitude.
Pour plus de détails : "Cooling a Bose Gas by ThreeBody Losses".Physical Review Letters, 121 , pp.200401 (2018)
The effective 1D interaction strength depends on the tranverse confinement of the atoms.Thanks to the versatility of our modulated guide, we can change the transverse confinement, independantly on the longitudinal one. We can thus realise quenches of the interaction strength of the 1D gas and we investigated the outofequilibrium dynamics following a sudden quench of the interaction strength. Within a linearized approximation, the system is described by independent collective modes (the Bogoliubov modes, or equivalently the modes of the LuttingerLiquid model) and the quench squeezes the phase space distribution of each mode, leading to a subsequent breathing of each quadrature.
We show that the collective modes are resolved by the power spectrum of density ripples which appear after a short time of flight. This allows us to experimentally probe the expected breathing phenomenon. Our results are in good agreement with theoretical predictions which take the longitudinal harmonic confinement into account.
For more information, see Physical Review A, 98, p.043604 (2018)
Ultracold temperatures are routinely obtained in cold atoms experiments using evaporative cooling. An energyselective loss process removes the most energetic atoms; provided these atoms have a high enough energy, rethermalization of the remaining atoms leads to a lower temperature. Evaporative cooling however becomes unefficient once the transverse degrees of freedom are frozen. Cooling then relies on a simple onebody loss process, as shown in the group of J. Schmiedmayer in Vienna (Phys. Rev. Lett. 116, 030402 (2016), Phys. Rev. A 93, 033634 (2016)). We showed that this cooling produces non thermal states, whose longlived nature is garantied by the integrability of the model of bosonic atoms with contact interactions. We also developp a MonteCamro wave function analysis of this cooling mecanism, which enable us to propose a quantum feedback scheme to cool to ground state one or several collective modes. Finally, we substantially extended previous work by investigating the effect of jbody losses, i.e. losses due to collosionnal process involving j atoms and where j atoms are lost at each event.
Cooling due to losses is seen on low energy collective modes, the phonon modes. The temperature of those modes results from a competition between two effetcs. On the one hand losses produce a reduction of the energy in each phonon mode since a smaller amplitude of density modulations lowers the interaction energy of each phonon mode. On the other hand, the shot noise due to the discrete nature of losses is responsible for an increase of the density fluctuations in the gas, and thus increases the energy in each mode. The competition between these two processes leads to a decay of the temperature such that the ratio between $k_B T$ and $mc^2$, where $m$ is the atomic mass and $c$ the speed of sound, becomes asymptotically a constant, of the order of unity.
For more detail see our publications :"Cooling phonon modes of a Bose condensate with uniform few body losses". SciPost Physics, vol.5 , p.043 (2018); "Longlived nonthermal states realized by atom losses in onedimensional quasicondensates", Physical Review A 96, pp.013623 (2017); "A Monte Carlo wavefunction description of losses in a 1D Bose gas and cooling to the ground state by quantum feedback." Physical Review A 95, pp.043641 (2017)
Analyzing the noise in the momentum profiles of single realizations of onedimensional Bose gases, we present the experimental measurement of the full momentumspace density correlations, which are related to the twobody momentum correlation function. Our data span the weakly interacting region of the phase diagram, going from the ideal Bose gas regime to the quasicondensate regime. We show experimentally that the bunching phenomenon, which manifests itself as superPoissonian local fluctuations in momentum space, is present in all regimes. The quasicondensate regime is, however, characterized by the presence of negative correlations between different momenta, in contrast to the Bogolyubov theory for Bose condensates, predicting positive correlations between opposite momenta. Our data are in good agreement with ab initio calculations : either simplified models valid ion the asymptotic regimes of Ideal Bose gas and quasicondensates respectively, or quantum Monte Carlo calculations performed by Tommaso Roscilde.
We investigated the breathing mode of quasicondensates both in real space and in momentum space. The profile in real space reveals sinusoidal width oscillations whose frequency varies continuously through the quasicondensate to ideal Bose gas crossover. In momentum space and for cold enough quasicondensates, we report the first observation of a frequency doubling phenomenon : the width of the momentum distribution shows two minima per breathing period, at the outer turning point when the realspace density distribution is the largest and at the inner turning point when the cloud is the thinnest. The narrowing of the momentum width at the inner turning point corresponds to a selfreflection mechanism due to the repulsive interactions. The disappearance of the frequency doubling as the temperature of the gaz is increased is mapped out experimentally.
In situ density fluctuation measurements were used to probe the different regimes of a repulsive 1D Bose gas. The repulsive interactions suppress bosonic bunching in the quasicondensate phase leading to a reduction of the density fluctuations. We mapped the phase diagram by tuning temperature and interaction strength. Density fluctuation measurements can also be used as a thermometry.
For weakly interacting gases (right), we observed fluctuations that are superpoissonian (due to bosonic bunching) at intermediate densities and which become subpoissonian at large density, in the quantum quasicondensate regime. At larger interaction strengths (left), the gas is close to the fermionised regime. Here the fluctuations are close to poissonian, a feature that resembles what is expected for a Fermi gas.


The signature of quasicondensation is not so sharp in momentum space and lorentzianlike distributions were observed on both sides of the crossover. The measured momentum distributions were compared to Quantum Monte Carlo calculations for the finitetemperature Lieb and Liniger model and the extracted temperatures were in agreement with in situ measurements. Momentum space brings complementary information compared to real space.
We are currently investigating the limits of dissipative cooling experimentaly.
We are currently working on an imaging system with better resolution and a new laser setup. We are also installing an optical lattice.
Experimental study of the outofequilibrium dynamics of 1D Bose gases
Sélection spatiale d’une partie d’un nuage d’atomes ultrafroids.
Spatial selection of a ultracold cloud
Group meetings take place every monday at 11:00 in Salle du conseil (2nd floor)
Institut d’Optique Graduate School, April 4th 2016.