  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 of the DQD. This derived excitation condition agrees well
with experiments on QPC-induced inelastic electron tunneling through a DQD [S.
Gustavsson et al., Phys. Rev. Lett. 99, 206804(2007)]. Moreover, we propose a
new scheme to generate a pure spin current by the QPC in the absence of a
charge current.
