Induced coherence with and without induced emission
  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. 
H.M. Wiseman, K. Molmer