Expansion of a Bose-Einstein Condensate with Vortices

The expansion of Bose-Einstein condensates (BECs) is numerically studied. Usually, the aspect ratio of a condensate is inverted due to the anisotropy of the uncertainty principle. In turbulent BECs, however, the aspect ratio is observed to remain constant. The reason for this phenomenon is still unknown, being a challenging topic in the study of quantum turbulence. Here, the two-dimensional Gross-Pitaevskii equation is solved and the time development of the aspect ratio and of the radii of the condensates are calculated in the presence of vortices. The results indicate that vortex pairs must be added to a condensate in order to expand it with keeping its aspect ratio. The anisotropy due to the vortex pairs competes with that due to the uncertainty principle so that the aspect ratio remains constant.