Vortex Excitations in a Bose-Einstein Condensate

We experimentally studied vortex excitations in a dilute gas Bose-Einstein condensate. First, the phase profile of a vortex excitation was observed using an interferometric technique. Vortices were created by moving a laser beam through a condensate, and they were observed as dislocations in the interference fringes formed by the stirred condensate and a second unperturbed condensate. Secondly, we have observed the formation of highly ordered vortex lattices in a rotating Bose-condensed gas. These triangular lattices contained over 100 vortices with lifetimes of several seconds. Individual vortices persisted up to 40 seconds. The gaseous Bose-Einstein condensates can be a model system for the study of vortex matter.