The GRAVITES fiber interferometer consists of three 27 km optical fibers wound on vibration-insensitive coils and vertically separated by approximately 20 m. Through careful mechanical design supported by finite element analysis, the coils exhibit two orders of magnitude reduction in sensitivity to external vibrations. The gravitational effect on our two-photon path-entangled states corresponds to an effective length difference of only about 10 pm between the coils. Detecting such a small signal is highly demanding due to both environmental and intrinsic noise sources. To address this, we employ a combination of state-of-the-art active and passive stabilization techniques, achieving a path-length stability better than 1 pm. Entangled photons are generated using a custom-built type-0 spontaneous parametric down-conversion source, delivering a spectral brightness of approximately 390,000 coincidences/mW/nm/s and a loss-corrected heralding efficiency exceeding 70%. Beyond vibrational noise, a further limitation arises from the relative propagation delay between photons traveling through different fibers due to Earth's rotation. To quantify this effect for entangled states of light, we performed an experiment with an effective enclosed area of 715 m². Our results show that, for suitable coil geometries and winding directions, the resulting delay can be reduced below the expected gravitational signal. To describe the influence of gravity on light propagation, we developed a comprehensive framework for quantum optics in curved spacetime. This model describes the propagation of single photons through optical fibers and predicts how their spectral properties are modified by gravitational fields. In parallel, we introduced a new geometric formalism that enables direct comparisons of quantum interferometry in external gravitational fields for both massive particles and photons in the massless limit. Our theoretical analysis further demonstrates that existing experimental results from selected optical waveguide experiments already exclude the possibility of a nonzero photon mass within the standard extensions of Maxwell's equations typically considered. We also investigated the elastic response of optical waveguides to varying gravitational fields, providing design constraints on allowable pressure and temperature fluctuations. Finally, we experimentally demonstrated sensitivity to artificially generated phase shifts with amplitudes comparable to the expected gravitational phase shifts on single-photon states. These measurements confirmed shot-noise-limited operation of the interferometer, validating the performance required for the GRAVITES experiment.
