Holly Middleton-Spencer (Newcastle University) gives a webinar on ‘Vortex States within Expanding Bose-Einstein Condensates’ at 4:30pm UK time

Zoom link: https://newcastleuniversity.zoom.us/j/89135861130

Meeting ID: 891 3586 1130

Passcode: 459726

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. A well-known example of this is that of a cigar-shaped condensate – as the condensate expands, it inverts its aspect ratio – a characteristic well established in the literature. It has, however, been observed that a turbulent condensate does not undergo this inversion. Thus, a self-similar expansion has been described as a tell-tale signature of turbulence within the condensate.

We present a talk in two parts. We begin by looking at reconnections during the expansion of the condensate. Reconnections of vortices are a fundamental part of quantum turbulence, instigating a cascade of the energy spectrum towards a turbulent state. It has been assumed that releasing a condensate from its confining potential will freeze the dynamics within it and that an image of the condensate after expansion will directly relate to that of the trapped state. We show how reconnections are affected by the expansion, and also show that they are still possible in the early-time expansion. We introduce a generalisation to the well-established relation of the evolution of the minimum distance of two reconnecting vortices for an expanding case. We further discuss the implications for any measurement for an experimental measure of energy cascades. Next, we look at lattice formations of vortices within cigar condensates and study the expansion of the overall system. We quantify the effect of said vortices and conclude that self-similar expansion can no longer be a signature of a turbulent state and further delve into the mechanism of vortices withholding the aspect ratio inversion of the expanding cloud.