Sitting in a trap at temperatures so low that their quantum wavefunctions overlap is a weird enough state for most atoms to be in — now scientists want to get them in a spin. Elsewhere in this issue D. A. Butts and D. S. Rokhsar (Nature 397, 327–329; 1999) predict what would happen if a Bose-Einstein condensate (a gas of atoms so cold and so dense that the atoms act as one) were made to rotate.

Bose-Einstein condensation is also responsible for superfluidity in liquid helium, which has an unusual response to rotation. Unlike normal fluids, circulating flow in liquid helium always produces vortices. But, understanding rotation in superfluid helium is not straightforward because of the strong interactions between atoms. The advantage of using condensate gases is that the atoms are weakly interacting, making them simpler to study.

Butts and Rokhsar calculate that rotating a Bose gas will produce a series of stable states as the gas spins faster and faster, with a pinwheel pattern emerging at higher velocities (see picture). The black dots represent zero density of the condensate and correspond to vortices: the gas flows anticlockwise about each vortex, as shown by the order of the rainbow colours around each dot. The central vortices form a crystal-like lattice, which is similar to patterns seen in rotating superfluid helium.

The most striking feature of the spinning condensates is their lack of full rotational symmetry, and these predicted ‘signatures’ should help experimentalists searching for vortices in Bose-Einstein condensates.