  The Hong-Ou-Mandel (HOM) effect is analyzed for photons in a modified
Mach-Zehnder setup with two particles experiencing different gravitational
potentials, which are later recombined using a beam-splitter. It is found that
the HOM effect depends directly on the relativistic time dilation between the
arms of the setup. This temporal dilation can be used to estimate the $\gamma$
and $\beta$ parameters of the parameterized post-Newtonian formalism. The
uncertainty in the parameters $\gamma$ and $\beta$ are of the order $
10^{-8}-10^{-12}$, depending on the quantum state employed.
