1. Department of Physics, University of California, Berkeley, California 94720-7300, USA and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Correspondence to: D. S. Rokhsar¹ Correspondence and requests for materials should be addressed to D.S.R. (e-mail: Email: rokhsar@physics.berkeley.edu).

Abstract

Superfluids are distinguished from normal fluids by their peculiar response¹ to rotation: circulating flow in superfluid helium²,³, a strongly coupled Bose liquid, can appear only as quantized vortices^{4, 5, 6}. The newly created Bose–Einstein condensates⁷,⁹—clouds of millions of ultracold, weakly interacting alkali-metal atoms that occupy a single quantum state—offer the possibility of investigating superfluidity in the weak-coupling regime. An outstanding question is whether Bose–Einstein condensates exhibit a mesoscopic quantum analogue of the macroscopic vortices in superfluids, and what its experimental signature would be. Here we report calculations of the low-energy states of a rotating, weakly interacting Bose gas. We find a succession of transitions between stable vortex patterns of differing symmetries that are in general qualitative agreement with observations⁵ of rotating superfluid helium, a strong-coupling superfluid. Counterintuitively, the angular momentum per particle is not quantized. Some angular momenta are forbidden, corresponding to asymmetrical unstable states that provide a physical mechanism for the entry of vorticity into the condensate.

1. Department of Physics, University of California, Berkeley, California 94720-7300, USA and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Correspondence to: D. S. Rokhsar¹ Correspondence and requests for materials should be addressed to D.S.R. (e-mail: Email: rokhsar@physics.berkeley.edu).