  We have created a long-lived (~ 40 s) persistent current in a toroidal
Bose-Einstein condensate held in an all-optical trap. A repulsive optical
barrier creates a tunable weak link in the condensate circuit, which can affect
the current around the loop. Superflow stops abruptly at a barrier strength
such that the local flow velocity exceeds a critical velocity. The measured
critical velocity is consistent with dissipation due to the creation of
vortex-antivortex pairs. This system is the first realization of an elementary
closed-loop atom circuit.
