Producing flow in racetrack atom circuits by stirring

We present a study of how macroscopic flow can be produced in Bose-Einstein condensates confined in a "racetrack"potential by stirring with a wide rectangular barrier. This potential consists of two half-circle channels separated by straight channels of length L and reduces to a ring potential if L=0. We present the results of a flow-production study where racetrack condensates were stirred with a barrier under varying conditions of barrier height, stir speed, racetrack geometry, and temperature. The result was that stirring was readily able to produce flow in ring and nonring geometries but that the exact amount of flow produced depended on all of the study parameters. We therefore investigated the mechanism by which flow was produced in the stirring process. The basic mechanism that we discovered was that when the sweeping barrier potential height reached a critical value a series of phase slip (i.e., a sudden change in the phase winding around the condensate midtrack) events occurred. Phase slipping stopped when the flow produced overtook the speed of the stirring barrier. Disturbances generated at each phase slip circulated around the channel and served to convert the initially localized velocity distribution into smooth macroscopic flow. This picture of the mechanism for making flow should facilitate the design of closed-channel atom circuits for creating a desired amount of quantized smooth flow on demand.