Figure 6

Frequency-entangled HOM interferometer sensitive to the gravitational redshift and consisting of a ground station and a spacecraft. (a) Simplified schematic. Photons experience different gravitational potentials, depending on which path they take, before recombination on a non-polarizing beam splitter (blue). (b) Overlapping-path architecture, with a single uplink channel for both photons; polarization entanglement ensures that only interfering processes are present, i.e., due to the polarizing beamsplitters (black squares, PBS), each photon takes one long and one short path. While the diagrams suggest that imperfect extinction ratios of the PBSs may lead to measurement errors, in fact these are strongly suppressed, since “wrong” coincidences can only occur if photons take the incorrect output of all three PBSs