Self-Consistent Stochastic Finite-Temperature Modelling: Ultracold Bose Gases with Local (s-wave) and Long-Range (Dipolar) Interactions

Abstract

We formulate a generalized self-consistent quantum kinetic theory including thermal fluctuations and stochastic contributions for modelling ultracold Bose gases interacting via a generic long-range interaction. Our generalised equations take the usual form of an effective field theory, separating coherent, low-lying, modes of the system from incoherent, higher-lying, thermal modes. The low-lying modes are described by a stochastic Langevin equation with two explicitly time-dependent collisional terms (corresponding to a dissipative and an energy-correcting contribution) and their corresponding additive and multiplicative stochastic noise terms. By coupling such an equation to an explicitly non-equilibrium gas of incoherent (thermal) particles described by a quantum Boltzmann equation, we thus extend beyond both earlier stochastic approaches (including the full SPGPE) and generalised kinetic models inspired by a two-gas picture (the so-called ZNG formalism) commonly used in the context of short-range interactions, such as those relevant in ultracold alkali atoms. Long-range interactions are further included into our model by the self-consistent addition of a Poisson-like equation for the long-range interaction potential. Our approach leads directly to a self-consistent model for finite-temperature Bose-Einstein condensation in a long-range interacting system within the regime where thermal fluctuations dominate over quantum fluctuations. While such an approach could be of general use for a variety of experimentally-accessible long-range interacting systems, we focus specifically here on the well-studied case of dipolar atomic condensates. In this particular context, we additionally supplement our Keldysh non-equilibrium analysis for fluctuations of the fast (incoherent) modes by a somewhat ad hoc extension of the slow (coherent) modes via the usual route of Bogoliubov-de Gennes equations.