Induced coherence and indistinguishability in optical interference
X. Y. Zou, L. J. Wang, L. Mandeļ · Physical Review Letters · 1991
Second-order interference is observed in the superposition of signal photons from two coherently pumped parametric down-converters, when the paths of the idler photons are aligned. The interference exhibits certain nonclassical features; it disappears when the idlers are misaligned or separated by a beam stop. The interpretation of this effect is discussed in terms of the intrinsic indistinguishability of the photon paths.