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 $\pi \rho {\alpha }_{j,j^{\prime }}\gtrsim 0.4$, where $\rho$ is the metal (Fermi sea) density of states, taken as a constant, and ${\alpha }_{j,j^{\prime }}$ is the scattering potential energy of electrons, between the $j$ and $j^{\prime }$ 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.

• Received 22 March 2013

DOI:https://doi.org/10.1103/PhysRevB.87.195436