Free expansion of quasi-2D Bose-Einstein condensates with quantized vortices

TL;DR

This study investigates the free expansion dynamics of quasi-2D Bose-Einstein condensates with vortices, revealing how vortex cores evolve into concentric ripples and how defocused imaging affects vortex visualization.

Contribution

It provides experimental and numerical analysis of vortex core evolution during free expansion, highlighting the effects of defocusing and vortex charge on imaging.

Findings

Vortex cores evolve into concentric density ripples during expansion.

Defocused images magnify vortex cores and can make them appear filled.

Expansion dynamics are well described by non-interacting simulations.

Abstract

We observe that a density-depleted vortex core evolves into concentric density ripples in a freely expanding quasi-two-dimensional Bose-Einstein condensate. The atomic density of the expanding condensate rapidly reduces due to the fast expansion along the tightly confining axial direction, and thus the transverse expansion of the condensate is not significantly affected by the atom-atom interactions. We find that the observed density profiles of the vortex cores are in good quantitative agreement with numerical simulations assuming no interactions. We analyze the defocused images of the vortex cores, where defocussing is caused by the free fall of the condensate during the expansion time. In the defocused images, the vortex core becomes magnified and appears to be filled after a certain expansion time which depends on the vortex charge number.