Master Equation for a Particle Coupled to a Two-Level Reservoir
  We study the quantum dissipative dynamics of a particle coupled linearly to a set of two-level systems (the heat bath) via the master equation method which we extract from the path integral formalism independently from the form of the bath spectral density. We compare our results with the standard models based on bosonic heat baths showing their main differences and similarities. In particular, we study special forms for the spectral density of the bath which give results quite different from the standard models. 
Paulo C. Marques and Antonio H. Castro Neto