Completely Positive Bloch-Boltzmann Equations
  The density operator of the arbitrary physical system must be positive definite. Employing the general master equation technique which preserves this property we derive equations of motion for the density operator of an active atom which interacts collisionally with the reservoir of perturber atoms. The obtained general relations applied to the two-level atom yield Bloch-Boltzmann equations (BBE). The form of the BBE obtained by us differs from that known from literature which, as we show, are not guaranteed to preserve the required positivity. We argue that our results are the correct ones and as such should be used in practical applications. Moreover, the structure and the terms which appear in our set of BBE seem to allow simpler and more straightforward physical interpretation. 
Robert Alicki, Stanis{\l}aw Kryszewski