Vortex states within expanding Bose-Einstein Condensates

Time of flight imaging is an important technique to visualize atomic Bose-Einstein condensates. As the condensate cloud is released from its confinement, a plethora of phenomena can be observed, for example, a cigar shape condensate will expand and invert its aspect ratio – a signature well established in the literature. It has, however, been observed that a turbulent condensate does not undergo this inversion, and thus a self-similar expansion has been described as a tell-tale signature of turbulence within the condensate. 

 

In this work we take a step back and study both how vortices affect the expansion of a condensate and how the expansion effects the vortices. We begin by considering a single vortex in harmonically trapped condensates of varying aspect ratios: we quantify the width and length of the vortex during the expansion and identify the change of the various contributions (interaction, compressible kinetic and incompressible kinetic) to the total energy of the expanding condensate. Then we move our attention to the reconnection of vortices within the condensate, a fundamental mechanism for the dynamics of the turbulence. We provide quantitative evidence that reconnections of simple vortex systems are effectively frozen in the early-time expansion of the condensate. This result confirms that the experimental images of an expanded turbulent condensates are snapshots of the previously evolving condensates.