A multi-site variational master equation approach to dissipative energy transfer

TL;DR

This paper introduces a variational master equation approach for open quantum systems, enabling accurate modeling of energy transfer in large exciton networks with complex environments beyond traditional approximations.

Contribution

It extends the variational (polaron) transformation to large, complex exciton networks, providing a unified framework that interpolates between weak coupling and full polaron regimes.

Findings

The method agrees with established master equations in their validity regimes.

It accurately predicts dynamics in parameter regimes where other methods struggle.

Provides new insights into the balance of coherent and incoherent energy transfer processes.

Abstract

Unitary transformations can allow one to study open quantum systems in situations for which standard, weak-coupling type approximations are not valid. We develop here an extension of the variational (polaron) transformation approach to open system dynamics, which applies to arbitrarily large exciton transport networks with local environments. After deriving a time-local master equation in the transformed frame, we go on to compare the population dynamics predicted using our technique with other established master equations. The variational frame dynamics are found to agree with both weak coupling and full polaron master equations in their respective regions of validity. In parameter regimes considered difficult for these methods, the dynamics predicted by our technique are found to interpolate between the two. The variational method thus gives insight, across a broad range of parameters, into the competition between coherent and incoherent processes in determining the dynamical behaviour of energy transfer networks.