Appendix I: propagation of a light beam in a nonlinear parametric medium, inducing a medium-assisted energy transfer between the input beam and the generation of signal and idler beams, hence the name three-wave mixing given to this phenomenon, which is first treated classically, then in a fully quantum way. One finds that, as in the case of fluorescence by spontaneous emission, the phenomenon of spontaneous parametric down conversion (or parametric fluorescence) requires a full quantum treatment, whereas parametric gain can be calculated semiclassically. It gives rise to entangled signal and idler photons as well as twin beams when one inserts the nonlinear medium in a resonant optical cavity (optical parametric oscillator) and to squeezing when the signal and idler modes are identical.