Predicted signatures of rotating Bose-Einstein condensates

Superfluids are distinguished from normal fluids by their peculiar response (1) to rotation: circulating flow in superfluid helium(2,3), a strongly cou pled Bose liquid, can appear only as quantized vortices(4-6). The newly cre ated Bose-Einstein condensates(7,9)- clouds of millions of ultracold, weakl y interacting alkali-metal atoms that occupy a single quantum state-offer t he possibility of investigating superfluidity in the weak-coupling regime, An outstanding question is whether Bose-Einstein condensates exhibit a meso scopic quantum analogue of the macroscopic vortices in superfluids, and wha t its experimental signature would be, Here we report calculations of the l ow-energy states of a rotating, weakly interacting Bose gas. We find a succ ession of transitions between stable vortex patterns of differing symmetrie s that are in general qualitative agreement with observations(5) of rotatin g superfluid helium, a strong-coupling superfluid, Counterintuitively, the angular momentum per particle is not quantized. Some angular momenta are fo rbidden, corresponding to asymmetrical unstable states that provide a physi cal mechanism for the entry of vorticity into the condensate.