Quantum Theory of a Steady State Laser
  A theory of Steady-State laser action is presented which loosely parallels the treatment by H. Frohlich on energy storage in biological systems. The principal lasing elements, taken as N bio level molecules and a single mode quantized radiation field are treated as a single quantum system. Laser action then is seen as a Bose-Einstein like condensation into the lowest energy mode of this correlated system when the pump power exceeds a certain critical value. 
Frederick W. Cummings and Michael T. Tavis
quant-ph