Proposal for an optical interferometric measurement of the gravitational redshift with satellite systems

The Einstein equivalence principle (EEP) underpins all metric theories of gravity. One of its key aspects is the local position invariance of nongravitational experiments, which is captured by the gravitational redshift. The iconic gravitational redshift experiment places two fermionic systems, used as clocks, in different gravitational potentials and compares them using the electromagnetic field. However, the electromagnetic field itself can be used as a clock, by comparing the phases acquired by two optical pulses propagating through different gravitational potentials. A fundamental point in the implementation of a space-based large-distance optical interferometric experiment is the suppression of the first-order Doppler effect, which dominates the weak gravitational signal necessary to test the EEP. Here, we propose a novel scheme to suppress it by subtracting the phase-shifts measured in a one-way and two-way configuration between a ground station and a satellite. We present a detailed analysis of this technique within the post-Newtonian framework and perform simulations of its performance using realistic satellite orbits and state-of-the-art fiber technology at the telecom wavelength of 1550 nm.