  The theoretical framework behind a recent experiment by Nogueira et. al.
[Phys. Rev. Lett. 86}, 4009 (2001)] of spatial antibunching in a two-photon
state generated by collinear type II parametric down-conversion and a
birefringent double-slit is presented. The fourth-order quantum correlation
function is evaluated and shown to violate the classical Schwarz-type
inequality, ensuring that the field does not have a classical analog. We expect
these results to be useful in the rapidly growing fields of quantum imaging and
quantum information.
