Two-photon interference in a Mach-Zehnder interferometer
John Rarity, P. R. Tapster, Eric Jakeman, Todd S. Larchuk, Richard A. Campos, M. C. Teich, Bahaa E. A. Saleh · Physical Review Letters · 1990
Pairs of 826.8-nm correlated photons, generated by parametrically down-converting 413.4-nm krypton-ion laser light, are directed into a single Mach-Zehnder interferometer such that each photon of the pair enters a different input port. The rate of coincidence at the two output ports displays oscillations (as the path-length difference is swept) with a spatial period equal to the 413-nm wavelength of the pump photons, and with a visibility of 62% when the path-length difference exceeds the coherence length of the individual photon beams. This unequivocally demonstrates the nonclassical and entangled nature of the two-photon state.