Phase coherence of a Bose-Einstein condensate in an optical antidot lattice

Abstract

We report on the experimental investigation of the phase coherence of a Bose-Einstein condensate in an optical antidot lattice (ADL). In the ADL, a series of potential hills are arranged on a two-dimensional (2D) square lattice and crisscrossing channels with zero ac Stark shift form a 2D mesh with multiple junctions. Due to these structural features, matter wave interference released from the ADL is clearly visible even at antidot heights of the 300 times the photon recoil energy. However, phase coherence is significantly reduced at extremely tall antidot heights and atom hopping is suppressed. We explain this loss of coherence by introducing a periodic effective potential felt by the atoms on the mesh lines, which is the result of the quantization of the faster atomic motion in the orthogonal direction. This effective potential localizes atoms at the junctions, forming a bundle of tubelike gases. Using the Bose-Hubbard model, we interpret the coherence properties in terms of a 2D Josephson junction array. We have also observed the emergence of vortexlike holes after merging the tubelike gases. We discuss the effect of transverse thermal phase fluctuations and interpret the appearance of the vortexlike holes as a possible signature of the Berezinskii-Kosterlitz-Thouless crossover in the 2D lattice plane.