Qubit-mediated energy transfer between thermal reservoirs: Beyond the Markovian master equation

We study qubit-mediated energy transfer between two electron reservoirs by adopting a numerically exact influence functional path-integral method. This nonperturbative technique allows us to study the system's dynamics beyond the weak coupling limit. Our simulations for the energy current indicate that perturbative-Markovian master equation predictions significantly deviate from exact numerical results already at intermediate coupling pi rho alpha (j, j') greater than or similar to 0.4, where rho is the metal (Fermi sea) density of states, taken as a constant, and alpha(j, j') is the scattering potential energy of electrons, between the j and j' states. Perturbative Markovian master equation techniques should be therefore used with caution beyond the strictly weak subsystem-bath coupling limit, especially when a quantitative knowledge of transport characteristics is desired.