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 ${\omega }_1$ and ${\omega }_2$, it is found that the joint probability of two-photon detection exhibits a modulation of the form $cos({\omega }_1-{\omega }_2)\tau$, where $c\tau$ is the path difference. The experimental results are well described by a simple quantum-mechanical analysis.

• Received 14 March 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.54