Observation of Irrotational Flow and Vorticity in a Bose-Einstein Condensate

We have observed the expansion of vortex-free Bose-condensates after their sudden release from a slowly rotating anisotropic trap and found clear experimental evidence for irrotational flow. At higher rotation frequencies we nucleated vortices (singularities in the macroscopic wavefunction of the condensate). Our experiment used a purely magnetic TOP trap. This time-averaged orbiting potential was rotated by modifying the bias field which allowed us to study the conditions for vortex nucleation over a wide range of eccentricities and rotation rates.