Dissipation in Open Two-Level Systems: Perturbation Theory and Polaron   Transformation
  We compare standard perturbation theory with the polaron transformation for non-linear transport of electrons through a two-level system. For weak electron-phonon coupling and large bias, there is good agreement between both approaches. This regime has recently been explored in experiments in double quantum dots. 
T. Brandes (UMIST, Manchester), T. Vorrath (Hamburg)