A local realist theory of parametric down conversion

In a series of articles we have shown that all parametric-down- conversion processes, both of type-I and type-II, may be described by a positive Wigner density. These results, together with our description of how light detectors subtract the zeropoint radiation, indicated the possibility of a completely local realist theory of all these processes. In the present article we show how the down-converted fields may be described as retarded fields, generated by currents inside the nonlinear crystal, thereby achieving such a theory. Most of its predictions coincide with the standard nonlocal theory. However, the intensities of the down converted signals do not correspond exactly with the photon pairs of the nonlocal theory. For example, in a blue- red down conversion we would find 1.03 red "photons" for every blue one. The theory also predicts a new phenomenon, namely parametric up conversion from the vacuum.