Induced Coherence, Vacuum Fields, and Complementarity in Biphoton   Generation
  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. 
A. Heuer, R. Menzel, P. W. Milonni
quant-ph