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Mesures de corrélations dans un gaz de bosons unidimensionnel sur puce

T. Jacqmin

Published Nov 22, 2012• 1 citation
Publisher

Abstract

In this manuscript, we present spatial one and two-body correlation measurements performed on a one-dimensional gas of ultra-cold bosons trapped at the surface of a microstructure. Two body correlations are highlighted by measurements of in situ density fluctuations and one-body correlations are probed through measurements of momentum distributions.We observed sub-Poissonian density fluctuations in the regime of weak interactions, thus demonstrating for the first time the regime of quasi-condensate in which the two-body correlation function is dominated by quantum fluctuations. We also observed sub-Poissonian fluctuations regardless of the density in the regime of strong interactions. Our measurement is the first observation of a single one-dimensional gas of bosons in this regime.The magnetic trap that we used is a modulated trap that has the remarkable property of decoupling between transverse and longitudinal confinements. This specificity has enabled us to engineer at will the shape of the longitudinal confinement. In particular, we were able to obtain harmonic and quartic traps.

DOI
10.70675/4ed2fb2dz57e3z44f5za261z470b9e5207bd

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In this manuscript, we present spatial one and two-body correlation measurements performed on a one-dimensional gas of ultra-cold bosons trapped at the surface of a microstructure. Two body correlations are highlighted by measurements of in situ density fluctuations and one-body correlations are probed through measurements of momentum distributions.We observed sub-Poissonian density fluctuations in the regime of weak interactions, thus demonstrating for the first time the regime of quasi-condensate in which the two-body correlation function is dominated by quantum fluctuations. We also observed sub-Poissonian fluctuations regardless of the density in the regime of strong interactions. Our measurement is the first observation of a single one-dimensional gas of bosons in this regime.The magnetic trap that we used is a modulated trap that has the remarkable property of decoupling between transverse and longitudinal confinements. This specificity has enabled us to engineer at will the shape of the longitudinal confinement. In particular, we were able to obtain harmonic and quartic traps.
We report on local, in situ measurements of atom number fluctuations in slices of a one-dimensional Bose gas on an atom chip setup. By using current modulation techniques to prevent cloud fragmentation, we are able to probe the crossover from weak to strong interactions. For weak interactions, fluctuations go continuously from super- to sub-Poissonian as the density is increased, which is a signature of the transition between the subregimes where the two-body correlation function is dominated, respectively, by thermal and quantum contributions. At stronger interactions, the super-Poissonian region disappears, and the fluctuations go directly from Poissonian to sub-Poissonian, as expected for a "fermionized" gas.
Dimensionality plays an important role in physical systems. A gas of ultracold atoms confined in a two-dimensional geometry has very different thermodynamic properties from those observed in a three-dimensional gas. In this thesis, we present experiments performed on a gas of ultracold bosons trapped in two dimensions and interacting weakly with each other. First, we describe this system with a mean-field approach and present the experimental setup that allows us to study it. Then, we describe two experiments dealing with two distinct topics, both conducted in the superfluid regime at near zero temperature. We first describe the creation of dimers by microwave-induced photoassociation of atoms trapped in a flat-bottom potential. This experiment allows us to characterise the interaction potential of our atomic species, rubidium 87. We also characterise the interactions between the dimers created and the bath of free atoms. In a third part, we impose a breaking of translational invariance on our zero-temperature gas of bosons by creating a spatially modulated potential in one of the two directions of the atomic cloud. We show with two distinct measurements that the superfluid density, equal to the total density for a homogeneous gas, decreases when the translational invariance is broken. We measure the sound velocity in the direction parallel and perpendicular to the spatial modulation on the one hand, and estimate the superfluid fraction according to a definition introduced by Leggett in 1970 on the other hand. We then observe a superfluid fraction which decreases as the modulation amplitude increases.
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 ⟨δn_{p}δn_{p^{'}}⟩, 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.
We report the first direct in situ observation of density fluctuations on the scale of the thermal de Broglie wavelength in an ultracold gas of bosons. Bunching of ^{87}Rb atoms in a quasi-two-dimensional system is observed by single-atom imaging using a quantum gas microscope. Compared to a classical ensemble, we observe a 30% enhancement of the second-order correlation function. We show the spatial and thermal dependence of these correlations. The reported method of detecting in situ correlations can be applied to interacting many-body systems and to the study of critical phenomena near phase transitions.
We present a quantum Monte Carlo study of the one-body density matrix (OBDM) and the momentum distribution of one-dimensional (1D) dipolar bosons, with dipole moments polarized perpendicular to the direction of confinement. We observe that the long-range nature of the dipole interaction has dramatic effects on the off-diagonal correlations: although the dipoles never crystallize, the system goes from a quasi-condensate regime at low interactions to a regime in which quasi-condensation is discarded in favor of quasi-solidity. For all strengths of the dipolar interaction we considered, the OBDM shows an oscillatory behavior coexisting with an overall algebraic decay, while the momentum distribution shows sharp kinks at the wavevectors of the oscillations, Q =±2π n (where n is the atom density), beyond which it is strongly suppressed. This momentum filtering effect introduces a characteristic scale in the momentum distribution, which can be arbitrarily squeezed by lowering the atom density. This shows that 1D dipolar Bose gases, realized e.g. by trapped dipolar molecules, show strong signatures of the dipolar interaction in time-of-flight measurements.