Multiple-path transport in quantum networks

We find an exact expression for the current (I) that flows via a tagged bond from a site (dot) whose potential (u) is varied in time. We show that the analysis reduces to that of calculating time-dependent probabilities, as in the stochastic formulation, but with splitting (branching) ratios that are not bounded within [0, 1]. Accordingly, our result can be regarded as a multiple-path version of the continuity equation. It generalizes results that have been obtained from adiabatic transport theory in the context of quantum 'pumping' and 'stirring'. Our approach allows us to address the adiabatic regime, as well as the slow and fast non-adiabatic regimes, on equal footing. We emphasize aspects that go beyond the familiar picture of sequential Landau-Zener crossings, taking into account the Wigner-type mixing of the energy levels.