Hysteresis in a quantized superfluid /`atomtronic/' circuit

TL;DR: The results suggest that the relevant excitations involved in hysteresis are vortices, and indicate that dissipation has an important role in the dynamics.

Abstract: Hysteresis is observed between circulation states in an ‘atomtronic’ circuit formed from a ring of superfluid Bose–Einstein condensate obstructed by a rotating weak link (a region of low atomic density), and may prove as crucial in future atomtronic devices as it has done in electronic devices. Hysteresis, a phenomenon by which the physical properties of a system depend strongly on the history of the applied perturbation, is widely exploited in electronic circuits including hard disk drives and flux-gate magnetometers and is essential to the function of radio-frequency SQUIDs (superconducting quantum interference devices). Hysteresis is also fundamental to superfluidity and has been predicted to occur in superfluid atomic-gases, such as Bose–Einstein condensates (BECs). Gretchen Campbell and colleagues now report the first direct detection of hysteresis between quantized circulation states in a circuit formed from a ring of superfluid BEC obstructed by a rotating weak link. The presence of hysteresis in this system is of importance in the emerging field of 'atomtronics', in which ultracold atoms have a role analogous to that of the electrons in electronics. Controlled hysteresis in atomtronic circuits may prove to be a crucial feature for the development of practical devices. Atomtronics1,2 is an emerging interdisciplinary field that seeks to develop new functional methods by creating devices and circuits where ultracold atoms, often superfluids, have a role analogous to that of electrons in electronics. Hysteresis is widely used in electronic circuits—it is routinely observed in superconducting circuits3 and is essential in radio-frequency superconducting quantum interference devices4. Furthermore, it is as fundamental to superfluidity5 (and superconductivity) as quantized persistent currents6,7,8, critical velocity9,10,11,12,13,14 and Josephson effects15,16. Nevertheless, despite multiple theoretical predictions5,17,18,19, hysteresis has not been previously observed in any superfluid, atomic-gas Bose–Einstein condensate. Here we directly detect hysteresis between quantized circulation states in an atomtronic circuit formed from a ring of superfluid Bose–Einstein condensate obstructed by a rotating weak link (a region of low atomic density). This contrasts with previous experiments on superfluid liquid helium where hysteresis was observed directly in systems in which the quantization of flow could not be observed20, and indirectly in systems that showed quantized flow21,22. Our techniques allow us to tune the size of the hysteresis loop and to consider the fundamental excitations that accompany hysteresis. The results suggest that the relevant excitations involved in hysteresis are vortices, and indicate that dissipation has an important role in the dynamics. Controlled hysteresis in atomtronic circuits may prove to be a crucial feature for the development of practical devices, just as it has in electronic circuits such as memories, digital noise filters (for example Schmitt triggers) and magnetometers (for example superconducting quantum interference devices).