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

In this talk, I will discuss our recent measurement of the gravitational attraction between cesium atoms in free fall and a centimeter-sized source mass using atom interferometry. Placing the source mass in vacuum provides sensitivity to a wide class of ``fifth force'' type interactions whose effects would otherwise be suppressed beyond detectability in regions of high matter density. Examples include so-called chameleon and symmetron fields, proposed as dark energy candidates. Our measurement tightens constraints on such theories by over two orders of magnitude.