  We investigate general properties of optical interferometry in stationary
spacetimes and apply the obtained results focusing on quantum-optical
experiments in near-Earth environments. We provide a rigorous expression for
the {gravitationally induced} phase difference and adapt the parametrized
post-Newtonian formalism for calculations of polarization rotation. We
investigate two optical versions of the Colella-Overhauser-Werner experiment
and show that the phase difference is independent of the post-Newtonian
parameter $\gamma$, making it a possible candidate for an optical test of the
Einstein equivalence principle. Polarization rotation provides an example of
the quantum clock variable, and while related to the optical Lense-Thirring
effects, shows a qualitatively different behaviour.
