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

Atomic interferometry measurements of the gravitational force on free-falling atoms provide improved constraints on certain scalar field theories trying to explain dark energy. Traditional gravity measurements use bulk masses to both source and probe gravitational fields1. Matter-wave interferometers enable the use of probe masses as small as neutrons2, atoms3 and molecular clusters4, but still require fields generated by masses ranging from hundreds of kilograms5,6 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 scale7 for a wide class of cosmologically motivated scalar field models8,9 of modified gravity and dark energy. We improve the limits on two such models, chameleons9 and symmetrons10,11, 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 effect12.