Cluster Truncated Wigner Approximation in Strongly Interacting Systems

TL;DR

This paper introduces a generalized truncated Wigner approximation method that efficiently simulates quantum dynamics in strongly interacting systems by approximating them with classical nonlinear dynamics, capturing quantum and thermal fluctuations.

Contribution

The paper develops a higher-dimensional phase space generalization of TWA that becomes asymptotically exact with larger clusters, improving accuracy over traditional methods.

Findings

Accurately simulates quantum dynamics in spin chains.

Captures phenomena like many-body localization and entanglement spread.

Demonstrates effectiveness in disordered and clean systems.

Abstract

We present a general method by which linear quantum Hamiltonian dynamics with exponentially many degrees of freedom is replaced by approximate classical nonlinear dynamics with the number of degrees of freedom (phase space dimensionality) scaling polynomially in the system size. This method is based on generalization of the truncated Wigner approximation (TWA) to a higher dimensional phase space, where phase space variables are associated with a complete set of quantum operators spanning finite size clusters. The method becomes asymptotically exact with the increasing cluster size. The crucial feature of TWA is fluctuating initial conditions, which we approximate by a Gaussian distribution. We show that such fluctuations dramatically increase accuracy of TWA over traditional cluster mean field approximations. In this way we can treat on equal footing quantum and thermal fluctuations as well as compute entanglement and various equal and non-equal time correlation functions. The main limitation of the method is exponential scaling of the phase space dimensionality with the cluster size, which can be significantly reduced by using the language of Schwinger bosons and can likely be further reduced by truncating the local Hilbert space variables. We demonstrate the power of this method analyzing dynamics in various spin chains with and without disorder and show that we can capture such phenomena as long time hydrodynamic relaxation, many-body localization and the ballistic spread of entanglement.