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Atom chip

Members

  • PhD students : Maximilian Schemmer
  • Former PhD students : Aisling Johnson, Bess Fang, Tarik Berrada as part time (co-tutelle with atom-chip group of Atomic Institute - Vienna University), Thibaut Jacqmin, Julien Armijo, Karim El-Amili, Jean-Baptiste Trebbia, Jérôme Estève, Thorsten Schumm, Christine Aussibal
  • Former Post-doc : Carlos Garrido-Alzar, Sébastien Gleyzes

!!! Students and Post-Doc welcome !!!

Introduction

Atom chip

We study the physics of one-dimensional Bose gases using an atom chip set-up. 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 quasi-condensates 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 many-body systems.

Toolbox

  • Imaging of real space density distribution (bottom left) and momentum distributions (right): The surface of the chip is used as a mirror to perform absorption imaging. A simple absorption image yields the in situ density distributions while the magnetic focusing technique allows to probe momentum distribution.
  • Analysis of density ripples (top left) that appear after a short time-of-flight to gain information on phase fluctuations initially present in the gas.
  • High control of the longitudinal and transverse potential: the longitudinal confinement is harmonic up to fifth order and transverse and longitudinal confinement can be tuned independantly.
  • Modulated guide: The current providing the transverse trapping potential is modulated at 200 kHz such that the atoms see a time-averaged potential, therefore reducing roughness due to fabrication imperfections.

imagedensityripples

In situ

Focus

Recent results:

  • Monitoring squeezed collective modes of a 1D Bose gas after an interaction quench

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 out-of-equilibrium 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 Luttinger-Liquid 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.archives-ouvertes.fr/hal-01661074

quenchg-osc

  • Dissipative cooling of a quasi-condensate

Ultra-cold temperatures are routinely obtained in cold atoms experiments using evaporative cooling. An energy-selective 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 one-body 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 long-lived nature is garantied by the integrability of the model of bosonic atoms with contact interactions. We also developp a Monte-Camro 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.

imageQMC

  • Momentum-Space Correlations of a One-Dimensional Bose Gas

Analyzing the noise in the momentum profiles of single realizations of one-dimensional Bose gases, we present the experimental measurement of the full momentum-space density correlations, which are related to the two-body 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 super-Poissonian 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 quasi-condensates respectively, or quantum Monte Carlo calculations performed by Tommaso Roscilde.

  • Self-reflexion mecanism in the breathing of a 1D quasicondensate

We investigated the breathing mode of quasi-condensates 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 quasi-condensates, 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 real-space 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 self-reflection mechanism due to the repulsive interactions. The disappearance of the frequency doubling as the temperature of the gaz is increased is mapped out experimentally.

breathing-small

  • Sub-poissonian density fluctuations in a repulsive 1D Bose gas

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 quasi-condensate 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 super-poissonian (due to bosonic bunching) at intermediate densities and which become sub-poissonian at large density, in the quantum quasi-condensate 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.

Atomic fluctuations close to the strong interaction regime

Atomic-fluctuations-for-weakly-interacting-gases_large

  • Momentum distribution at the quasi-condensation crossover:

The signature of quasi-condensation is not so sharp in momentum space and lorentzian-like distributions were observed on both sides of the crossover. The measured momentum distributions were compared to Quantum Monte Carlo calculations for the finite-temperature Lieb and Liniger model and the extracted temperatures were in agreement with in situ measurements. Momentum space brings complementary information compared to real space.

Ongoing work:

  • Dissipative cooling of a quasi-condensate

We are currently investigating the limits of dissipative cooling experimentaly.

  • Improvement of the experiment:

We are currently working on an imaging system with better resolution and a new laser setup. We are also installing an optical lattice.

Internship proposal:

Experimental study of the out-of-equilibrium dynamics of 1D Bose gases

Sélection spatiale d’une partie d’un nuage d’atomes ultra-froids.

Spatial selection of a ultra-cold cloud

List of publications

  • Long-Lived nonthermal states realized by atom losses in one-dimensional quasicondensates. Aisling Johnson, Stuart S. Szigeti, Maximilian Schemmer and Isabelle Bouchoule, Phys. Rev. A 96, 013623 (2017)
  • Monte CArlo wave-function description of losses in a one-dimensionnal Bose gas and cooling to the ground state by quantum feedback. Maximilian Schemmer, Aisling Johnson, R. Photopoulos and I. Bouchoule, Phys. Rev. A 95, 043641 (2017)
  • Momentum Space Correlations of a One-Dimensional Bose Gas.
    Bess Fang, Aisling Johnson, Tommaso Roscilde, and Isabelle Bouchoule, Phys. Rev. Lett. 116, 050402
  • Quench-Induced Breathing Mode of One-Dimensional Bose Gases
    Bess Fang, Giuseppe Carleo, Aisling Johnson, and Isabelle Bouchoule, Physical Review Letters 113 (2014) 053301
  • Momentum distribution of one-dimensional Bose gases at the quasicondensation crossover: Theoretical and experimental investigation
    Thibaut Jacqmin, Bess Fang, Tarik Berrada, Tommaso Roscilde, and Isabelle Bouchoule, Physical Review A 86 (2012) 043626
  • Two-body momentum correlations in a weakly-interacting one-dimensional Bose gas
    Bouchoule I, Arzamazovs M, Kheruntsyan K.V., Gangardt D.M.
    Physical Review A: Atomic, Molecular and Optical Physics (2012) vol. 86, p. 033626
  • Sub-Poissonian fluctuations in a 1D Bose gas: from the quantum quasi-condensate to the strongly interacting regime
    Jacqmin T., Armijo J., Berrada T., Kheruntsyan K., Bouchoule I.
    Physical Review Letters 106 (2011) 230405
  • Mapping out the quasi-condensate transition through the 1D-3D dimensional crossover
    Armijo J., Jacqmin T., Kheruntsyan K., Bouchoule I.
    Physical Review A: Atomic, Molecular and Optical Physics (2011) vol. 83, p. 021605
  • Probing three-body correlations in a quantum gas using the measurement of the third moment of density fluctuations
    Armijo J., Jacqmin T., Kheruntsyan K., Bouchoule I.Physical Review Letters 105 (2010) 230402
  • Thermal properties of AlN-based atom chips Armijo J., Garrido Alzar C., Bouchoule I. European Physical Journal D 56 (2009
  • Limitation of the modulation method to smooth wire guide roughness Bouchoule I., Trebbia J.-B., Garrido Alzar C. Physical Review A: Atomic, Molecular and Optical Physics 77 (2008) 023624
  • Roughness suppression via rapid current modulation on an atom chip Trebbia J.-B., Garrido Alzar C., Cornelussen R., Westbrook C. I., Bouchoule I. Physical Review Letters 98, 263201 (2007)
  • Observations of density fluctuations in an elongated Bose gas: ideal gas and quasi-condensate regimes Estève J., Trebbia J.-B., Schumm T., Aspect A., Westbrook C. I., Bouchoule I. Physical Review Letters 96 (2006) 130403
  • Experimental evidence for the breakdown of a Hartree-Fock approach in a weakly interacting Bose gas Trebbia J.-B., Estève J., Westbrook C. I., Bouchoule I. Phys. Rev. Lett. 97 (2006)
  • Realizing a stable magnetic double-well potential on an atom chip Estève J., Schumm T., Trebbia J.-B., Bouchoule I., Aspect A., Westbrook C. I. European Physical Journal D 35 (2005)
  • Atom chips in the real world: the effects of wire corrugation Schumm T., Estève J., Aussibal C., Figl C., Trebbia J.-B., Nguyen H., Mailly D., Bouchoule I., Westbrook C. I., Aspect A. European Physical Journal D 32 (2005)
  • The role of wire imperfections in micro magnetic traps for atoms Estève J., Aussibal C., Schumm T., Figl C., Mailly D., Bouchoule I., Westbrook C. I., Aspect A. Physical Review A: Atomic, Molecular and Optical Physics 70 (2004)

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