  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.
