We report the results of two experiments performed with two-photon light,
produced via collinear degenerate optical spontaneous parametric downconversion
(SPDC), in which both second-order (one-photon) and fourth-order (two-photon)
interferograms are recorded in a Mach-Zehnder interferometer (MZI). In the
first experiment, high-visibility fringes are obtained for both the second- and
fourth-order interferograms. In the second experiment, the MZI is modified by
the removal of a mirror from one of its arms; this leaves the fourth-order
interferogram unchanged, but extinguishes the second-order interferogram. A
theoretical model that takes into consideration both the temporal and spatial
degrees-of-freedom of the two-photon state successfully explains the results.
While the temporal interference in the MZI is independent of the spatial
coherence of the source, that of the modified MZI is not