The atomic Bose gas in Flatland

TL;DR

This paper reports an experiment with a 2D Bose gas of rubidium atoms, observing a BKT crossover characterized by a loss of coherence and vortex proliferation, confirming theoretical predictions.

Contribution

First experimental observation of BKT transition in a 2D atomic Bose gas using matter wave heterodyning.

Findings

Observation of quasi-coherence at low temperatures

Detection of vortex proliferation at higher temperatures

Confirmation of BKT theory predictions

Abstract

We describe a recent experiment performed with rubidium atoms ($^{87}$Rb), aiming at studying the coherence properties of a two-dimensional gas of bosonic particles at low temperature. We have observed in particular a Berezinskii--Kosterlitz--Thouless (BKT) type crossover in the system, using a matter wave heterodyning technique. At low temperatures, the gas is quasi-coherent on the length scale set by the system size. As the temperature is increased, the loss of long-range coherence coincides with the onset of the proliferation of free vortices, in agreement with the microscopic BKT theory.