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When signal and idler photons produced in the process of parametric down-conversion are mixed together, and directed to two photodetectors that respond to nonoverlapping optical frequencies centered at ₁ and ₂, it is found that the joint probability of two-photon detection exhibits a modulation of the form cos (₁-₂), where c is the path difference. The experimental results are well described by a simple quantum-mechanical analysis.
Ou et al. (Mon,) studied this question.