We propose methods to perform intensity interferometry of photons having twodifferent wavelengths. Distinguishable particles typically cannot interferewith each other, but we overcome that obstacle by processing the particles sothat they lead to the same final state at the detection apparatus.Specifically, we discuss how quasi-phase-matched nonlinear crystals can be usedto convert a quantum superposition of light of different wavelengths onto acommon wavelength, while preserving the phase information essential for theirmeaningful interference. We generalize the van Cittert-Zernike formula for theintensity interferometry of extended sources, demonstrate how our proposalsupports enhanced resolution of sources with different spectral character, andsuggest potential applications.
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