But in most standard situations a wave function is defined in terms of amplitudes of possible experimental outcomes. So shouldn't this be true for multi-photon states as well?
Suppose we have a bi-photon |2V,0H> i.e. both photons are vertically polarized, and none is polarized horizontally. Can Quantum Optics predict the amplitude of all 4 possible pairs of outcomes RR, LL, RL, LR if one measures both photons in the same (e.g. circular) basis?