Superfluid Dissipation and Feedback Cooling in Ultracold Atomic Gases
Creators: name: Mehdi, Zain
Publisher: name: The Australian National University
Year: 2024

description: Ultracold atomic gases are versatile and controllable platforms for modern investigations into many-body quantum science, quantum information, and quantum metrology. Today, ultracold atomic physics faces two key challenges. Firstly, as the technical capabilities and ambitions of cold-atom experiments continue to grow, so too does the demand for comprehensive theoretical modelling that can faithfully account for the many-body dynamics of the interacting atomic cloud, particularly in non-equilibrium regimes where beyond-mean-field effects cannot be neglected. Secondly, there is an urgent need to improve upon the cooling techniques used to produce ultracold gases, which largely follow the same procedure pioneered in the mid-1990s (laser cooling, followed by evaporation). This thesis contributes theoretical investigations addressing these two areas of ultracold atomic physics. The first half of this thesis presents two investigations in which we theoretically model non-equilibrium superfluid phenomena in finite-temperature Bose gases for which a quantitative description remains lacking, despite previous theoretical and experimental work. In the first of these investigations, we perform detailed simulations of an experiment exploring temperature-dependent superflow decay in a toroidal Bose-Einstein condensate (BEC), within the framework of c-field theory. The predictions of our ab initio numerical calculations provide a quantitative description of experimentally-observed decay timescales at low temperatures where quantum fluctuations dominate over thermal effects. At higher temperatures, we observe increasing discrepancies between the simulations and experimental data, suggesting the need for further experimental and theoretical studies into superflow stability. Next, we develop a microscopic open-quantum-systems theory of thermally-damped vortex motion in oblate atomic superfluids, that includes previously neglected energy-damping interactions between superfluid and thermal atoms. We derive an analytic expression for the mutual friction coefficient that gives excellent quantitative agreement with experimentally measured values, without any fitted parameters, closing an existing two orders of magnitude gap between experiment and theory. This work establishes the microscopic mechanisms underpinning the mutual friction and diffusion of two-dimensional quantized vortices in degenerate Bose gases. In the second half of this thesis, we present novel theoretical models of feedback-controlled atomic gases and use these to assess the feasibility of efficiently feedback-cooling quantum gases to degeneracy. Specifically, we consider an experimentally-realistic scheme in which atomic density fluctuations are monitored non-destructively by a far-detuned optical field, and controlled in real-time using high-resolution, high-bandwidth spatiotemporal potentials. We first consider the feedback cooling of a high-energy thermal gas towards quantum degeneracy, for which we establish fundamental limits of cooling set by the visibility of density fluctuations, measurement-induced heating, and three-body atomic recombination. Our results establish the fundamental viability of using feedback cooling to produce large atom-number quantum gases in low-dimensional geometries, beyond what is achievable in current experiments. Next, we consider the feedback cooling of collective modes of oscillation in a low-energy BEC, for which we develop an analytically tractable model of the multi-mode system dynamics under a realistic control scheme. We demonstrate the feasibility of multi-mode cooling the collective excitations close to their ground states, and explore the trade-off between cooling speed and the final energy of the cloud. Together, these investigations quantify the experimental conditions and feedback protocols needed for efficient feedback cooling and support the viability of realising feedback-cooled BECs in the near future.