As well as linear and angular momentum. The latter is the most important quantities for quantum optics because that is what produces polarization correlations.
This is what I presumed that the output is photons with lower frequencies but is it splitting in sense of really separating parts of the photon or just transfer to lower energy state that for conservation of energy produces two new photons while consuming the first one?
The photons have no inner parts. This is more similar to destroying the photon and creating three new photons.
(I don't like "absorbing". They use probably a non linear crystal, and the transformation is probably not in a single spot. I'm not sure about the details, but probably the photon slowly disappear while the photon is traveling inside the crystal, and at the same time the other three photons slowly apear. This must be interpreted as probabilities of seen the original photons or the new three photons at the other end of the crystal. You can't have half a photon, only a 50% probability of seeing a whole photon. This explanation has too many weird things and handwaving ... just use "destroying" instead of "absorbing". More info https://en.wikipedia.org/wiki/Spontaneous_parametric_down-co... )
As well as linear and angular momentum. The latter is the most important quantities for quantum optics because that is what produces polarization correlations.