Reconciling the Classical-Field Method with the Beliaev Broken Symmetry Approach

TL;DR

This paper discusses the strengths and limitations of classical-field methods versus symmetry-breaking approaches in modeling degenerate Bose gases, highlighting the potential for extending c-field methods to describe complex non-equilibrium phenomena.

Contribution

It provides a critical comparison of classical-field and symmetry-breaking methods, emphasizing the advantages of c-field approaches in non-equilibrium dynamics of Bose gases.

Findings

c-field methods can describe beyond-mean-field physics

symmetry-breaking approaches have limitations in non-equilibrium regimes

c-field methods can potentially be extended to include collective oscillations

Abstract

We present our views on the issues raised in the chapter by Griffin and Zaremba [A. Griffin and E. Zaremba, in Quantum Gases: Finite Temperature and Non-Equilibrium Dynamics, N. P. Proukakis, S. A. Gardiner, M. J. Davis, and M. H. Szymanska, eds., Imperial College Press, London (in press)]. We review some of the strengths and limitations of the Bose symmetry-breaking assumption, and explain how such an approach precludes the description of many important phenomena in degenerate Bose gases. We discuss the theoretical justification for the classical-field (c-field) methods, their relation to other non-perturbative methods for similar systems, and their utility in the description of beyond-mean-field physics. Although it is true that present implementations of c-field methods cannot accurately describe certain collective oscillations of the partially condensed Bose gas, there is no fundamental reason why these methods cannot be extended to treat such scenarios. By contrast, many regimes of non-equilibrium dynamics that can be described with c-field methods are beyond the reach of generalised mean-field kinetic approaches based on symmetry-breaking, such as the ZNG formalism.