Dissipation in a superfluid atom circuit

Abstract

Bose-Einstein condensates of weakly interacting dilute atomic gases provide a unique system with which to study phenomena associated with superfluidity. The simplicity of these systems allows us to study the fundamental physics of superfluidity without having to consider the strong interactions present in other superfluid systems such as superconductors and liquid helium. While condensate-based studies have been around for 20 years, our novel approach to confining ultracold atoms has opened a completely new range of parameter space to investigate. Armed with an ability for straightforward creation of arbitrary, time-dependent potential landscapes in which to study superfluid interactions, we were able to take a closer look at predictions of superfluid behavior that are decades old, but until now have never been tested directly. The purpose of this research was to draw direct analogies between superfluid BEC systems, which we term superfluid atom circuits, and existing superconducting circuits, thus allowing us to take advantage of much of the existing knowledge that has come from this well-studied field. Specifically, existing circuits and devices that have been created with superconductors give us insight into what might be possible someday with atom-circuit devices and inspiration to create them. In these experiments, we employed two different atom circuits; one classical (thermal ideal gas) and one quantum (ultracold superfluid). Our results show that each system is equivalent to an electronic circuit consisting of a capacitor being discharged through an inductor in series with some dissipative element. In the thermal system, dissipation can be described in terms of simple resistive flow with the resistance equivalent to ballistic, Sharvin resistance seen in electronic circuits. The superfluid measurements show that the dissipation is best described as a resistance-shunted Josephson junction, which is an analogue to similar devices in superconducting circuits. Additionally, the specific geometry of the atom circuit we used in our superfluid system allowed us to investigate directly a predicted mechanism responsible for the dissipation in superfluids caused by the generation of collective excitations, namely vortices. Direct observation of this mechanism has not previously been possible in superfluid helium and superconducting systems.