Dynamics of hot Bose-Einstein condensates: stochastic Ehrenfest relations for number and energy damping

Abstract

Describing high-temperature Bose gases poses a long-standing theoretical challenge. We present exact stochastic Ehrenfest relations for the stochastic projected Gross-Pitaevskii equation, including both number and energy damping mechanisms, and all projector terms that arise from the energy cutoff separating system from reservoir. Analytic solutions for the center of mass position, momentum, and their two-time correlators are in close agreement with simulations of a harmonically trapped prolate system. The formalism lays the foundation to analytically explore experimentally accessible hot Bose-Einstein condensates. Contents arXiv:1908.05809v1 [cond-mat.quant-gas] B Kohn mode projector terms 21 B.1 Position and momentum 22 B.2 Dimensionless variable z(t) 23 B.3 Numerical evaluation of projector terms 23 References 24 IntroductionA system of Bose atoms with temperature T undergoes a dramatic change in behavior at the critical temperature for the formation of a Bose-Einstein condensate (BEC), T c . Far below the BEC transition T T c , a nearly pure BEC forms, consisting of a highly occupied many-body quantum state; in this regime dilute gas BEC are renowned both for their high experimental control, and precise theoretical description [1]. At temperatures T T c thermal energy dominates, the quantum statistics are unimportant, and a Boltzman description captures the physical properties of the atoms. When T ∼ T c the quantum statistics of the atoms are decisive, despite appreciable thermal energy. In a hot BEC T T c , competition between thermal and interaction effects leads to fragmentation of the condensate, and formation of vortices, solitons, and phononic excitations. A cooling quench across the transition can inject interesting excitations into the BEC that form as remnants of the broken U(1) symmetry [2, 3], and rich turbulent dynamics develop from the competition between thermal, quantum, and interaction effects, posing a challenge for theory.At low temperatures T T c , mean field theory provides a useful description, upon which the GPE and its generalizations are based. The Zaremba-Nikuni-Griffin U(1) symmetry breaking approach has proven well suited for practical calculations in the low-temperature regime [4], having the virtue that the interactions between condensate and thermal cloud, and their respective dynamics, are all included in the dynamical description. However, it's strength for low temperatures presents a limitation at high temperatures: the symmetry-breaking ansatz cannot describe strongly fluctuating systems containing large non-condensate fraction. Fortunately, the scope of the Gross-Pitaevskii equation (GPE) upon which ZNG is based goes much further than mean-field theory: GPE-like field equations appear naturally in phase-space representations of Bose gases [5], suggesting a generalization of quantum optical open systems theory [6] for describing hot Bose gases. Indeed, various generalized Gross-Pitaevskii theories have been developed for the high temperature regime, describing many modes that...

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