  We describe a novel technique to measure photon pair joint spectra by
detecting the time-correlation beat note when non-degenerate photon pairs
interfere at a beamsplitter. The technique, which we call auto-heterodyne
characterization (AHC), allows a high resolution frequency-difference spectrum
to be obtained with a passive optical system and photon counting. It implements
a temporal analog of the Ghosh-Mandel effect with one photon counter and a
time-resolved Hong-Ou-Mandel interference with two. With this technique, we
characterize the difference-frequency spectrum of photon pairs with a bandwidth
$\approx$ 5 MHz and frequency separation of 250 MHz from a cavity-enhanced
parametric downconversion source capable of addressing the hyperfine structure
of cold atomic Rb. The frequency resolution of the technique is effectively
unlimited, and the frequency range limited only by the resolution of the
detectors and timing electronics.
