Source-radiation field relation for two-photon emission of a two-level atom☆

Abstract A relation between a source and a radiation field, which is far away from a two-level atom, for two-photon emission is derived. Our result shows that the effective, positive part of the frequency of the radiation field is not only directly proportional to the atomic operator, σ(t − r c ) , but also depends on the exponential factor exp [− i (Ω′ − ω l )(t − r c )] , which determines the correct frequency relation in two-photon emission, and the photon creation operators of other modes in the initial radiation field, a λm † (Ω′ − ω l ) .