50-km Single-Photon Fiber Interferometer for Sensing Gravitational Signatures

Quantum mechanics and general relativity are foundational pillars of modern physics, yet experimental tests that directly connect the two remain rare. Measuring gravity-induced optical phase shifts of single photons offers a unique quantum platform for probing gravity beyond Newtonian descriptions, but laboratory interferometers have not yet reached the sensitivity required to access this regime. In this talk, I will present the realization of a 50-km tabletop Mach-Zehnder fiber interferometer operating at the single-photon level. We achieve a phase sensitivity of 4.42E-6 over the low frequency band 0.01-5 Hz. We further demonstrate that this sensitivity is sufficient to resolve a modulated phase signal of 6.18E-5 rad RMS at 0.1Hz. These results establish a milestone for large-scale optical interferometry in the single-photon regime, demonstrating a viable path toward detecting gravitational redshift signatures in a local laboratory, opening opportunities to test quantum phenomena in general-relativistic settings.