Beyond mean-field dynamics of small Bose-Hubbard systems based on the number-conserving phase space approach

TL;DR

This paper introduces a number-conserving phase space approach for small Bose-Hubbard systems that surpasses mean-field methods, accurately capturing dynamical instabilities, chaos, and higher moments of quantum states.

Contribution

It develops a phase space description based on SU(M) coherent states that extends beyond mean-field theory, enabling efficient simulation of large systems and resolving known limitations.

Findings

Accurately describes dynamical instabilities and chaos in Bose-Hubbard models.

Provides a semi-classical tool for higher moments and condensate depletion analysis.

Shows that fixed particle number approaches avoid artificial fluctuations present in other methods.

Abstract

The number-conserving quantum phase space description of the Bose-Hubbard model is discussed for the illustrative case of two and three modes, as well as the generalization of the two-mode case to an open quantum system. The phase-space description based on generalized SU(M) coherent states yields a Liouvillian flow in the macroscopic limit, which can be efficiently simulated using Monte Carlo methods even for large systems. We show that this description clearly goes beyond the common mean-field limit. In particular it resolves well-known problems where the common mean-field approach fails, like the description of dynamical instabilities and chaotic dynamics. Moreover, it provides a valuable tool for a semi-classical approximation of many interesting quantities, which depend on higher moments of the quantum state and are therefore not accessible within the common approach. As a prominent example, we analyse the depletion and heating of the condensate. A comparison to methods ignoring the fixed particle number shows that in this case artificial number fluctuations lead to ambiguities and large deviations even for quite simple examples.