We describe spontaneous parametric down-conversion experiments in which induced coherence across two coupled interferometers results in high-visibility single-photon interference. Opening additional photon channels allows "which-path" information and reduces the visibility of the singlephoton interference, but results in nearly perfect visibility when photons are counted in coincidence. A simplified theoretical model accounts for these complementary observations and attributes them directly to the relations among the vacuum fields at the different crystals.
@article{arxiv.1409.5696,
title = {Induced Coherence, Vacuum Fields, and Complementarity in Biphoton Generation},
author = {A. Heuer and R. Menzel and P. W. Milonni},
journal= {arXiv preprint arXiv:1409.5696},
year = {2015}
}