  We study the influence of the relativistic redshift on Hong-Ou-Mandel (HOM)
interference, and present a genuine quantum test of general relativity. We use
Glauber's theory of quantum coherence to predict the coincidence probability of
realistic broadband photons in HOM-experiments in a non-relativistic setting.
We extend the quantum field theoretical framework previously developed to
describe the deformation of the spectral profile of single photons in curved
spacetimes to a multi-photon framework, which is exact for inertial observers
in a flat spacetime and an approximation when observers are located in a curved
spacetime. We find that, in case of frequency entangled photons, a mutual
redshift between the sender and the receiver can change the coincidence
statistics from photon bunching to photon anti-bunching, and vice versa. This
implies that the (anti-) symmetry of the photonic spectral wave function is an
observer dependent notion, and that this can be probed via HOM-experiments in a
relativistic setting.
