To illustrate the exotic properties of "photons", it has become fashionable to use pairs generated by PDC (as we shall call it) instead of the earlier generation of atomic-cascade pairs. They are produced by causing a coherent laser beam to be incident on a nonlinear optical crystal. If the frequency of the laser is in the near ultraviolet (wavelength about 300 nanometres), then the light which emerges takes the form of a conical rainbow of visible light (wavelengths in the range 400-800 nm)
This phenomenon is currently generally known as parametric down conversion, but in my opinion this is an incorrect name, because it reflects the widely held view that a pair of visible photons are generated by the spontaneous decay of an ultraviolet photon from the laser. We obtain a more correct view of this process, once we realize that the nonlinear crystal brings about an interaction between the laser field and the zeropoint field, and that as a consequence of this interaction, there is a secondary field emitted by the crystal. In this way we are able to understand, in an entirely classical way, the real physical process which is occurring inside the crystal. My preferred name for this process is parametric amplification of the vacuum. Recently we have predicted that the intensities of the waves emitted in PDC do not exactly correspond to those of the Photon pair-creation theory. For example, if the laser pump has a wavelength of 267nm, then there is a correlation between the light emitted at 400nm ("blue") and at 800nm ("red"), as in the Photon theory. But, according to our theory, the intensities, translated into "photons", give 1.03 red photons for every blue one. We have also proposed an experimental test, and, if our prediction is confirmed, we think our colleagues will, at last, be obliged to accept our view that photons are now obsolete!References
Some of the peculiar properties of PDC "photons" are described in the following articles. Note that, in the second of these, the authors themselves use the description "mind-boggling", which may be taken as a synonym for magical.
A straightforward and scientific, that is nonmagical, explanation of these and other properties may be found in the following references: