We analyze signal coherence in the setup of Wang, Zou and Mandel, where two optical downconverters have indistinct idler modes. Quantum interference, caused by indistinguishability of paths, has a visibility proportional to the transmission amplitude between idlers. Classical interference, caused by induced emission, may be complete for any finite transmission.
@article{arxiv.quant-ph/0001118,
title = {Induced coherence with and without induced emission},
author = {H. M. Wiseman and K. Molmer},
journal= {arXiv preprint arXiv:quant-ph/0001118},
year = {2009}
}