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

We have observed the expansion of vortex-free, rotating Bose condensates after their sudden release from a slowly rotating anisotropic trap. Conservation of angular momentum, combined with the constraint of irrotational flow, cause the rotating condensate to expand in a distinctively different way to one released from a static (nonrotating) trap. This difference provides clear experimental evidence of the purely irrotational velocity field associated with a superfluid. We observed this behavior in absorption images taken along the rotation axis.