Abstract
We theoretically investigate the stochastic decay of persistent currents in a toroidal ultracold atomic superfluid caused by a perturbing barrier. Specifically, we perform detailed three-dimensional simulations to model the experiment of Kumar et al. in [Phys. Rev. A 95 021602 (2017)], which observed a strong temperature dependence in the timescale of superflow decay in an ultracold Bose gas. Our ab initio numerical approach exploits a classical-field framework that includes thermal fluctuations due to interactions between the superfluid and a thermal cloud, as well as the intrinsic quantum fluctuations of the Bose gas. In the low-temperature regime our simulations provide a quantitative description of the experimental decay timescales, improving on previous numerical and analytical approaches. At higher temperatures, our simulations give decay timescales that range over the same orders of magnitude observed in the experiment, however, there are some quantitative discrepancies that are not captured by any of the mechanisms we explore. Our results suggest a need for further experimental and theoretical studies into superflow stability.
Highlights
The initial conditions for our simulations of Eq (2) are stochastic samples of the C region’s quantum state at thermal equilibrium in the grand canonical ensemble. It is common for finite-temperature studies in the c-field framework to neglect quantum fluctuations in the initial state, on the assumption that they are dominated by thermal fluctuations in the hightemperature regime where stochastic projected Gross-Pitaevskii equation (SPGPE) is typically applied
This behaviour is to be contrasted with the superflow decay observed both in the experiment of Kumar et al [15] and the finitetemperature simulations conducted by Mathey et al [32], where stochastic decay events were observed at both short and long time scales, across the entire time period where the barrier was held at its maximum
The results from a single trajectory of the SPGPE are shown in Fig. 4, in which a decay event occurs roughly 110ms after the barrier reaches its maximum height
Summary
Ultracold atomic Bose gases are versatile, highly configurable systems, in part due to their isolation from environmental effects, precise controllability with magnetic, optical, and rf fields, and the accessible imaging of many atomic observables [1,2]. There have been many recent theoretical works that have investigated superfluidity and persistent currents in one-dimensional systems [20,21,22,23,24,25], protocols for atomic-gas superfluid circuits [26,27], superflow in dipolar supersolids [28], and mechanisms for superflow decay, both within mean-field theory [29,30,31] and beyond [32, 33]. Our model goes beyond mean-field theory and includes both the inherent fluctuations of the multimode three-dimensional quantum state, and finite-temperature interactions with an incoherent thermal reservoir These features are essential in order to describe the strong temperature-dependence of superflow decay observed in the experiment. At higher temperatures the simulations require a larger perturbing barrier height than the experiment to achieve the experimentally-observed decay timescales This discrepancy is largest for the highest tem-. We have explored some possibilities for the noted discrepancies in this work, the precise origin of this effect remains unclear
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