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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. 


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 preprint : https://hal.archivesouvertes.fr/hal01661074
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.
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.