Feynman integral in regularized non-relativistic quantum electrodynamics
  We express the unitary time evolution in non-relativistic regularized quantum electrodynamics at zero and positive temperature by a Feynman integral defined in terms of a complex Brownian motion. An average over the quantum electromagnetic field determines the form of the quantum mechanics in an environment of a quantum black body radiation. In this non-perturbative formulation of quantum electrodynamics we prove the existence of the classical limit $\hbar \rightarrow 0$.We estimate an error to some approximations commonly applied in quantum radiation theory. 
Z. Haba