Phase shifts vs time delays: Sagnac and Hong-Ou-Mandel
  We point out that the Sagnac effect can be measured by means of the Hong-Ou-Mandel effect. The latter is not sensitive to phase shifts, and thus the Hong-Ou-Mandel Sagnac effect hinges on the fact that the Sagnac effect is, fundamentally, a time delay, not a phase shift. 
S.J. van Enk
physics.optics quant-ph