Testing local position invariance with atom interferometry

Tests of local position invariance (LPI) with atom interferometers have gained considerable attention as a fundamental verification of the Einstein equivalence principle. So far, these discussions mainly focused on the universality of gravitational redshift (UGR), one of the two facets of LPI. Although UGR can be tested with spatially delocalized quantum-clock interferometry based on atom interferometers, such tests fall significantly short relative to conventional clock-based tests due to their limited duration and spatial separation. However, LPI manifests also via the universality of clock rates (UCR), predicting that two different clocks at the same height measure the same proper time. We show that whereas UCR tests in a fountain-clock Ramsey sequence are limited by initial conditions, quantum-clock interferometers with a cubic scaling in time circumvent this shortcoming. These schemes can be operated with optical frequencies and are robust against several parasitic effects. Since UCR is independent of spatial separations, the most prominent advantage of conventional clocks does not apply. We discuss different implementations which may outperform current UCR tests with fountain clocks.