Operating a quantum pump in a closed circuit

During an adiabatic pumping cycle a conventional two barrier quantum device takes an electron from the left lead and ejects it to the right lead. Hence the pumped charge per cycle is naively expected to be $Q\le e$. This zero order adiabatic point of view is in fact misleading. For a closed device we can get $Q>e$ and even $Q\gg e$. In this paper a detailed analysis of the quantum pump operation is presented. Using the Kubo formula for the geometric conductance, and applying the Dirac chains picture, we derive practical estimates for~$Q$.