The fiber interferometer for GRAVITES consists of two 27-kilometer-long optical fibers wound on vibration-insensitive coils that can be vertically separated by up to two meters with a moving stage. Through careful mechanical design optimized by finite element analysis, our coils show a hundred-fold reduction in sensitivity to external vibrations and our translation stage is capable of rapid height modulation with minimal disturbance to the interferometer. The effect of gravity on our two-photon path entanglement states corresponds to an induced length difference between the coils of about 10 picometers, which is extremely difficult to detect due to ambient and intrinsic noise. To combat them, we use a mixture of cutting edge active and passive stabilization techniques, which in combination provide us with a length stability of better than one picometer. To generate the entanglement, we are building single photon sources based on both spontaneous parametric down conversion and solid state quantum dots supplying more than 10 million entangled states per second. Another important effect that may limit our performance is the relative time delay between the propagation of light along the two different fibers, which is caused by the Earth's rotation. We performed an experiment with an effective area of 715 square meters to quantify its strength for entangled states of light. The results confirm that its influence can be smaller than the expected gravitational signal for a certain geometrical arrangement and winding direction of the two fiber coils. As light propagates through a solid material in standard optical fibers, it effectively samples the molecular motion of its constituents, which is another undesirable effect. For this reason, we investigated how hollow-core optical waveguides, a new type of fiber technology, behave in comparison. Our results show a high-fidelity distribution of entangled photons over kilometer-long distances as well as a strongly reduced thermal noise floor. To understand how gravity affects the propagation of light, we developed a comprehensive model of quantum optics in curved space-times, which models the propagation of single photons through optical fibers, and determines how their spectral properties are modified by the gravitational field. We have also shown in our calculations that the current experimental evidence for some waveguides already rules out the possibility that photons are massive by the standard modifications of Maxwell's equations that are typically considered. The elastic properties of optical waveguides in changing gravitational fields were determined to guide the experimental design and limit the maximum allowable pressure and temperature fluctuations.
