We study the Berezinskii-Kosterlitz-Thouless (BKT) crossover in a two-dimensional (2D) array of Josephson-coupled Bose-Einstein condensates. Applicable to many 2D systems, BKT theory predicts a phase transition between a low-temperature superfluid phase and the normal state at high temperature, mediated by thermally activated unbinding of vortex-antivortex pairs.In our lattice system, tunneling between nearest-neighbor condensates provides a Josephson coupling J which acts to keep the condensates' relative phases locked, while thermal fluctuations act to randomize the phases. As long as the Josephson energy J exceeds the thermal energy T, the array is vortex-free. With decreasing J/T we observe thermally activated vortices, allowing a â€œmicroscopicâ€ glimpse of the BKT crossover.
10 Minute Talk: Coherence revival in Atom-optics kicked rotor