||(a) In a donut, shaped, or 'toroidal' trap, atoms mostly exist in a red ring, and do not reside in the center (blue region), which represents an energy hill they cannot climb. (b) Image of a Bose-Einstein condensate (BEC) in the donut trap. (c) When there is no fluid flow around the donut, and the trap is turned off, atoms (red) rush to the center. (d) When fluid flows around the donut, and the trap is turned off, the current around the donut persists, and does not rush to fill the hole.
See also http://www.nist.gov/public_affairs/techbeat/tb2007_1127.htm#bec.