Feynman, Wigner, and Hamiltonian Structures Describing the Dynamics of   Open Quantum Systems
  This paper discusses several methods for describing the dynamics of open quantum systems, where the environment of the open system is infinite-dimensional. These are purifications, phase space forms, master equation and liouville equation forms. The main contribution is in using Feynman-Kac formalisms to describe the infinite-demsional components. 
J.E. Gough, T.S. Ratiu, O.G. Smolyanov
math-ph math.MP quant-ph