Direct observation of irrotational flow and evidence of superfluidity in a rotating Bose-Einstein condensate

G. Hechenblaikner, E. Hodby, S. A. Hopkins, O. M. Marago', C. J. Foot

Published 2001-06-10Version 1

We observed the expansion of vortex-free Bose-condensates after their sudden release from a slowly rotating anisotropic trap. Our results show clear experimental evidence of the irrotational flow expected for a superfluid. The expansion from a rotating trap has strong features associated with the superfluid nature of a Bose-condensate, namely that the condensate cannot at any point be cylindrically symmetric with respect to the axis of rotation since such a wavefunction cannot possess angular momentum. Consequently, an initially rotating condensate expands in a distinctively different way to one released from a static trap. We report measurements of this phenomenon in absorption images of the condensate taken along the direction of the rotation axis.