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.
Highlights
The transport modelThe system (S) considered in this work is that of N coupled two-level systems, known as sites
Unitary transformations can allow one to study open quantum systems in situations for which standard, weak-coupling type approximations are not valid
We can see from the various terms in (15) exactly how the parameters in the Hamiltonian enter into the system dynamics and the relative magnitude of these terms can give us an idea of the parameter regime of a given system—in the sense of which quantities are contributing most to the dynamics
Summary
The system (S) considered in this work is that of N coupled two-level systems, known as sites. Are electronic excitations which, for charge neutral systems, are called excitons. A good measure of the strength of the system–environment coupling at each site is the reorganization energy [25]: λn =. This model ignores any spatial correlations between phonon excitations at different sites, meaning that the Hamiltonian in (1) is not relevant for systems with strong, long-range correlations, such as impurities in BECs. For the case of FMO it has been claimed, based on detailed molecular dynamics simulations, that spatial correlations do not play a significant role in the exciton dynamics [49].
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