Effective nonlinear susceptibilities in the quantum theory of parametric down-conversion
Roger Andrews, Edward R. Pike, Sarben Sarkar · Physical Review A · 1994
The alternative multimode theories of parametric down-conversion based on either nonlinear susceptibility models or the general quantum electrodynamic interaction are critically examined and shown not to be equivalent. The predictions can be qualitatively different; in contrast to the former, within the framework of the latter, it is necessary to define more than one type of nonlinear susceptibility in order to describe one-photon and two-photon count rates.