Causal signal transmission by quantum fields. Electromagnetic interaction of distinguishable devices and the optical equivalence theorem

Generalised phase-space techniques for electromagnetic interactions beyond the rotating wave approximation [L.P. and S.S., arXiv:1104.3825 (2011)] is applied to interactions of distinguishable devices. The paper is built around the concept of "doing quantum electrodynamics while thinking classically," which is a generalisation of Sudarshan's renowned optical equivalence theorem [E.C.G. Sudarshan, Phys. Rev. Lett. 10, 277 (1963)] to the interacting electromagnetic field. This concept allows one to reduce inherently quantum problems to semiclassical considerations.