Testing sub-gravitational forces on atoms from a miniature in-vacuum source mass

Traditional gravity measurements use bulk masses to both source and probe gravitational fields(1). Matter-wave interferometers enable the use of probe masses as small as neutrons(2), atoms(3) and molecular clusters(4), but still require fields generated by masses ranging from hundreds of kilograms(5,6) to the entire Earth. Shrinking the sources would enable versatile configurations, improve positioning accuracy, enable tests for beyond-standard-model ('fifth') forces, and allow observation of non-classical effects of gravity. Here we detect the gravitational force between freely falling caesium atoms and an in- vacuum, miniature (centimetre-sized, 0.19 kg) source mass using atom interferometry. Sensitivity down to gravitational strength forces accesses the natural scale(7) for a wide class of cosmologically motivated scalar field models(8,9) of modified gravity and dark energy. We improve the limits on two such models, chameleons(9) and symmetrons(10,11), by over two orders of magnitude. We expect further tests of dark energy theories, and measurements of Newton's gravitational constant and the gravitational Aharonov-Bohm effect(12).