Exact derivation of the Langevin and master equations for harmonic   quantum Brownian motion
  A many particle Hamiltonian, where the interaction term conserves the number of particles, is considered. A master equation for the populations of the different levels is derived in an exact way. It results in a local equation with time-dependent coefficients, which can be identified with the transition probabilities in the golden rule approximation. A reinterpretation of the model as a set of coupled harmonic oscillators enables one to obtain for one of them an exact local Langevin equation, with time-dependent coefficients. 
Edgardo T. Garcia Alvarez and Fabian H. Gaioli