Dynamics and thermodynamics of strongly driven Bose gases

Abstract

This thesis explores ultracold Bose gases trapped in a uniform potential and forced far from equilibrium with a continuous drive. It contains experimental results obtained with a 39K gas, alongside numerical simulations and theoretical modeling. The starting point of the experiments is always an equilibrium (quasi-)pure condensate, and we explore both the dynamics of the system after turning on the drive and the ‘thermodynamics’ of the late-time steady-states, with an ultimate goal of developing a thermodynamics-like framework for far-from-equilibrium systems. The thesis consists of two main parts, which in turn explore weakly- and non-interacting Bose gases under continuous forcing. Firstly, we consider an interacting gas, and investigate the problem both experimentally (probing the full quantum problem) and numerically (within the classical field approximation using Gross-Pitaevskii equation simulations). Here, the continuous energy input leads to a turbulent cascade, which reaches a steady-state with matching energy injection and dissipation rates. The turbulent state, although steady, is far from thermodynamic equilibrium, but we find that it can still be described by an equilibrium-like equation of state (EoS), with new (far-from-equilibrium) state variables. We show that the EoS is universal in both the classical field approximation and in the full quantum case, but the two EoSs are different, and the quantum one cannot be described by any classical field model. We then turn to the case of the non-interacting gas, exploring the problem experimentally and theoretically, and find that the dynamics is governed by an interplay of the drive and the naturally present disorder. We show that this interplay results in an energy-space random walk and dynamically scalable momentum distributions. The observations for the non-interacting gas are different from the interacting case, but we demonstrate that there is a crossover between the disorder-dominated and interactions-dominated dynamics.