  Several recent papers have shown that some forms of dispersion cancellation
have classical analogs and that some aspects of nonlocal two-photon
interferometry are consistent with local realistic models. It is noted here
that the classical analogs only apply to local dispersion cancellation
experiments [A.M. Steinberg et al., Phys. Rev. Lett. 68, 2421 (1992)] and that
nonlocal dispersion cancellation [J.D. Franson, Phys. Rev. A 45, 3126 (1992)]
is inconsistent with any classical field theory and has no classical analog.
The local models that have been suggested for two-photon interferometry are
shown to be local but not realistic if the spatial extent of the
interferometers is taken into account. It is the inability of classical models
to describe all of the relevant aspects of these experiments that distinguishes
between quantum and classical physics, which is also the case in Bell's
inequality.
