# 2D rubidium Bose gases

Research**Loading and compression of a single uniform 2D Bose gas in an optical accordion
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We implemented on our new setup an optical accordion creating a lattice potential with a spacing that can be dynamically tuned between 11 and 2 µm. Atoms are loaded into a single node of this optical lattice in the large spacing configuration which is then decreased almost adiabatically to reach a strong harmonic confinement with a frequency larger than 10 kHz. This allows us to tune the effective 2D interaction strength. An additional flat-bottom in-plane potential is shaped with a high resolution thanks to a spatial light modulator (Digital Micromirror Device) combined with a microscope objective.

**References :**

J.L. Ville, T. Bienaimé, R. Saint-Jalm, L. Corman, M. Aidelsburger, L. Chomaz, K. Kleinlein, D. Perconte, S. Nascimbène, J. Dalibard, J. Beugnon

Phys. Rev. A 95, 013632 (2017) and arXiv:1611.07681

*Loading and compression of a single 2D Bose gas in an optical accordion*

**Bose gases in box potentials: investigating Kibble-Zurek mechanism
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We studied the formation of topological defects when quench cooling Bose gases. Atoms are trapped either in a square box-like potential or in a ring. In the box potential we observed the stochastic formation of vortices (see picture) and in the ring geometry we observed the creation of superfluid currents along the ring. Using matter-wave interference techniques we were able to measure the charge and direction of these currents. We also studied in the box potential the emergence of coherence due to the transverse condensation phase transition. We compared the rate of formation of these topological defects with Kibble-Zurek mechanism predictions.

**References :**

L. Chomaz, L. Corman, T. Bienaimé, R. Desbuquois, C. Weitenberg, S. Nascimbène, J. Beugnon, J. Dalibard

Nature Commum. 6, 6162 (2015) and arXiv:1411.3577

*Emergence of coherence in a uniform quasi-two-dimensional Bose gas*

L. Corman, L. Chomaz, T. Bienaimé, R. Desbuquois, C. Weitenberg, S. Nascimbène, J. Dalibard, and J. Beugnon

Phys. Rev. Lett. 113, 135302 (2014) and arXiv:1406.4073

*Quench-Induced Supercurrents in an Annular Bose Gas*

**Superfluid behavior of a 2D Bose gas**

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We demonstrated the superfluid behavior of a 2D Bose gas confined in a harmonic trap. By rotating a defect created by a repulsive laser beam we measured a threshold velocity for heating the cloud. We observed that this threshold is only present when we rotate in the superfluid inner region and not in the thermal outer region.

**References :**

R. Desbuquois, L. Chomaz, T. Yefsah, J. Léonard, J. Beugnon, C. Weitenberg, J. Dalibard

Nature Physics 8, 645 (2012) and arXiv:1205.4536

*Superfluid behaviour of a two-dimensional Bose gas*

**Equation of state of the weakly-interacting 2D Bose gas
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We measured the equation of state of a 2D Bose gas and compared it with theoretical predictions. The method is based on the measurement of the *in situ *density profile of the cloud trapped in a harmonic potential and the use of the local density approximation. We first used a method based on fitting the density profile to extract the chemical potential and the temperature. We also demonstrated a fit-free method to determine the equation of state.

**References :**

R. Desbuquois, T. Yefsah, L. Chomaz, C. Weitenberg, L. Corman, S. Nascimbène, J. Dalibard

Phys. Rev. Lett. 113, 020404 (2014) and arXiv:1403.4030

*Fit-free determination of scale invariant equations of state: application to the two-dimensional Bose gas*

T. Yefsah, R. Desbuquois, L. Chomaz, K. J. Günter, J. Dalibard

Phys. Rev. Lett. 107, 130401 (2011) and arXiv:1106.0188

*Exploring the thermodynamics of a two-dimensional Bose gas*

S.P. Rath, T. Yefsah, K.J. Günter, M. Cheneau, R. Desbuquois, M. Holzmann, W. Krauth, J. Dalibard

Phys. Rev. A 82, 013609 (2010) and arXiv:1003.4545

*The equilibrium state of a trapped two-dimensional Bose gas*