Resolution of the colocation problem in satellite quantum tests of the universality of free fall

A major challenge common to all Galilean drop tests of the universality of free fall (UFF) is the required control over the initial kinematics of the two test masses upon release due to coupling to gravity gradients and rotations. In this work, we consider a space-borne test of the UFF based on atom interferometry and show that this detrimental effect can be mitigated at the 10−18 level given an initial differential position (velocity) uncertainty in the order of μm (μm/s) of the test masses. This corresponds to a relaxation of the source control by several orders of magnitude with respect to comparable mission scenarios, such as the STE-QUEST mission proposal reported in [D. N. Aguilera et al., Classical Quantum Gravity 31, 115010 (2014)]. Our twofold mitigation strategy extends a compensation mechanism that is already established in terrestrial experiments to satellite missions with varying gravity gradients and exploits the spectral distribution of the systematics. We assess the experimental feasibility and find that the moderate parameters of the proposed scheme are in line with technological capabilities. The described attenuation of the gravity-gradient-induced uncertainty removes one major obstacle in quantum tests of the UFF and allows us to consider mission scenarios with target accuracies beyond the state of the art.