  We propose methods to perform intensity interferometry of photons having two
different wavelengths. Distinguishable particles typically cannot interfere
with each other, but we overcome that obstacle by processing the particles via
entanglement and projection so that they lead to the same final state at the
detection apparatus. Specifically, we discuss how quasi-phase-matched nonlinear
crystals can be used to convert a quantum superposition of light of different
wavelengths onto a common wavelength, while preserving the phase information
essential for their meaningful interference. We thereby gain access to a host
of new observables, which can probe subtle frequency correlations and
entanglement. Further, we generalize the van Cittert-Zernike formula for the
intensity interferometry of extended sources, demonstrate how our proposal
supports enhanced resolution of sources with different spectral character, and
suggest potential applications.
