The Inner Limit of Quantum Brownian Evolution and its Relevance to   Positivity
  The conventional quantum Brownian propagator, which describes the evolution of a system of interest bilinearly coupled to and initially uncorrelated with a reservoir, does not preserve positivity of density operators, prompting workers to modify the propagator by the ad hoc addition of time-independent terms to the corresponding generator. We show that no such terms need be added to the generator to preserve positivity provided one accounts for the rapid entanglement of the system of interest and the reservoir on a time scale too short for the conventional propagator to be valid. 
Allan Tameshtit