Parth Sabharwal’s thesis work on thermal phase fluctuations in narrow superfluid rings has appeared in today’s issue of Physical Review A.

Persistent currents in ultracold gases – ScienceDirect

J. Polo, W.J. Chetcuti, T. Haug, A. Minguzzi, K. Wright, L. Amico

Persistent currents flowing in spatially closed tracks define one of the most iconic concepts in mesoscopic physics. They have been studied in solid-state platforms such as superfluids, superconductors and metals. Cold atoms trapped in magneto-optical toroidal circuits and driven by suitable artificial gauge fields allow us to study persistent currents with unprecedented control and flexibility of the system’s physical conditions. Here, we review persistent currents of ultracold matter. Capitalizing on the remarkable progress in driving different atomic species to quantum degeneracy, persistent currents of single or multicomponent bosons/fermions, and their mixtures can be addressed within the present experimental know-how. This way, fundamental concepts of quantum science and many-body physics, like macroscopic quantum coherence, solitons, vortex dynamics, fermionic pairing and BEC-BCS crossover can be studied from a novel perspective. Finally, we discuss how persistent currents can form the basis of new technological applications like matter-wave gyroscopes and interferometers.

Parth Sabharwal has completed an extensive study of thermal phase fluctuations appearing in narrow superfluid rings, testing some interesting predictions made by a colleague Ludwig Mathey over a decade ago, and we have submitted a manuscript describing his work to Physical Review A.

Thermal Phase Fluctuations in Narrow Superfluid Rings (arXiv)

Using matter-wave interference, we have investigated thermal phase fluctuations in narrow coplanar, concentric rings of ultracold fermionic superfluids. We found that the correlation length decreases with number density, consistent with theoretical expectations. We also observed that increasing the coupling between the rings leads to greater overall coherence in the system. The phase fluctuations increased with a change from periodic to closed boundary conditions as we applied a potential barrier at one point in a ring. These results are relevant for the implementation of proposals to utilize ultracold quantum gases in large and elongated circuit-like geometries, especially those that require deterministic preparation and control of quantized circulation states.

Our recent work led by Daniel Allman on spontaneous currents in rings of fermionic superfluid following a rapid quench has now been published in Physical Review A

Quench-induced spontaneous currents in rings of ultracold fermionic atoms

Abstract: We have observed the spontaneous appearance of currents in a ring of ultracold fermionic atoms (Li-6) with attractive interactions, following a quench to a BCS-like pair superfluid. We have measured the winding number probability distribution for a range of quench rates, with a quench protocol using simultaneous forced evaporation and interaction ramps to achieve faster effective quench rates with less atom loss than a purely evaporative quench. We find that for the fastest quenches the mean-square winding number of the current follows a scaling law in the quench rate with exponent sigma=0.24(2) which is somewhat lower than that predicted by the Kibble-Zurek mechanism for the three-dimensional XY model (1/3) and unexpectedly closer to the value obtained from mean-field theory (1/4). For slower quenches nonuniversal effects become significant and we observe a lower rate of spontaneous current formation that does not follow a simple scaling law.

Daniel Allman has successfully defended his thesis “Equilibrium and Quench-Dynamical Studies of Ultracold Fermions in Ring-Shaped Optical Traps” and earned his Ph.D. The flagship result of his work is a study of quench-induced spontaneous currents in rings of fermionic superfluid, with a manuscript that we have submitted to Physical Review A currently under review. We have also learned quite a few things about phase fluctuations recently that Parth Sabharwal is going to be busy following up on for a while. Many thanks to Luigi Amico, Alex Rimberg, and James Whitfield for serving as members of Dan’s thesis committee.