Backaction of a charge detector on a double quantum dot

We develop a master equation approach to study the backaction of quantum-point contact (QPC) on a double quantum dot (DQD) at zero bias voltage. We reveal why electrons can pass through the zero-bias DQD only when the bias voltage across the QPC exceeds a threshold value determined by the eigenstate energy difference in the DQD. This derived excitation condition agrees well with experiments on QPC-induced inelastic electron tunneling through a DQD. Moreover, we propose a scheme to generate a pure spin current by the QPC in the absence of a charge current.