A Bose-condensed atomic Na gas, held in a toroidal trap, constitutes a (trivial) closed circuit—-the analog of a ring of superconductor. Flow induced in such a toroid persists “indefinitely,” i.e., for a time determined by the lifetime of the condensate, as limited by the imperfect vacuum in which it is held. We have introduced a variably permeable barrier into the ring so as to controllably interrupt the superflow and have measured the critical conditions at which the flow breaks. By rotating the barrier azimuthally around the ring we create the analog of a magnetic field applied to a superconducting loop with a weak link or Josephson junction. The behavior of the circuit is partly analogous to that of an electrical SQUID, and displays step-like behavior in the circulating current as a function of rotation rate as well as hysteresis.
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