Phase-space procedure for calculating multitime correlation functions of the electromagnetic field
Paul D. Lett, Rupamanjari Ghosh, Stephen R. Friberg, L. Mandeļ · Physical Review A · 1984
It is pointed out that a method recently described by Hillery and Zubairy can be generalized to yield the multitime correlation function in any order for an interacting quantum field in the form of a phase-space average. The correlations are expressible as $c$-number integrals with the help of the coherent-state propagator, but the associated phase-space density is complex.