Superflow in Bose-Einstein Condensate Rings: Tunable Weak Links in Atom Circuits

Persistent currents are a hallmark of both superfluidity and superconductivity. Just as a current in a superconducting circuit will flow forever, if a current is created in a superfluid Bose-Einstein condensate, the flow will not decay as long as the current is below a critical value.   We have created long-lived persistent currents in a toroidal-shaped Bose-Einstein Condensate, and studied the behavior of the current in the presence of both stationary and rotating weak links. A repulsive optical barrier across one side of the torus creates the tunable weak link in the condensate circuit and can be used to control the current around the loop. With a rotating weak link, at low rotation rates, we have observed phase slips between well-defined persistent current states.  This behavior is analogous to that of a weak link in a superconducting loop. A feature of our system is the ability to dynamically vary the weak link, which in turn varies the critical current, a feature which is difficult to implement in superconducting circuits. For higher rotation rates, we observe a transition to a regime where vortices penetrate the bulk of the condensate. These results demonstrate an important step toward realizing an atomic SQUID analog.