Semiclassical vs. quantum behavior in fourth-order interference

We explore semiclassical and quantum descriptions of fourth-order interference that are built upon classical and quantum Gaussian-state models for the signal and idler outputs from a parametric down converter. Our approach unifies the analysis of fourth-order interference with those for squeezing and photon twins. It also demonstrates that a purely-quantum fourth-order interference effect can be claimed at fringe visibilities substantially below the commonly accepted threshold of 50%. We start with a pair of models - one quantum and one classical - for the signal and idler fields produced by parametric down conversion