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is an emerging interdisciplinary field that seeks new functionality by creating devices and circuits where ultra-cold atoms, often superfluids, play a role analogous to the electrons in electronics. Hysteresis is widely used in electronic circuits, e.g., it is routinely observed in superconducting circuits 3 and is essential in rf-superconducting quantum interference devices [SQUIDs] 4 . Furthermore, hysteresis is as fundamental to superfluidity 5 (and superconductivity) as quantized persistent currents [6][7][8] , critical velocity [9][10][11][12][13][14] , and Josephson effects 15,16 . Nevertheless, in spite of multiple theoretical predictions 5,[17][18][19] , hysteresis has not been previously observed in any superfluid, atomic-gas Bose-Einstein condensate (BEC).Here we demonstrate hysteresis in a quantized atomtronic circuit: a ring of superfluid BEC obstructed by a rotating weak link. We directly detect hysteresis between quantized circulation states, in contrast to superfluid liquid helium experiments that observed hysteresis directly in systems where the quantization of flow could not be observed 20 and indirectly in systems that showed quantized flow 21,22 . Our techniques allow us to tune the size of the hysteresis loop and to consider the fundamental excitations that accompany hysteresis. The 1 arXiv:1402.2958v2 [cond-mat.quant-gas]

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