Population inversion of driven two-level systems in a structureless bath
  We derive a master equation for a driven double-dot damped by an unstructured phonon bath, and calculate the spectral density. We find that bath mediated photon absorption is important at relatively strong driving, and may even dominate the dynamics, inducing population inversion of the double dot system. This phenomenon is consistent with recent experimental observations. 
T. M. Stace, A. C. Doherty and S. D. Barrett