Atom-Interferometric Test of the Equivalence Principle at the 10^-12 Level

We use a dual-species atom interferometer with 2 s of free-fall time to measure the relative acceleration between ⁸⁵Rb and ⁸⁷Rb wave packets in the Earth's gravitational field.  Systematic errors arising from kinematic differences between the isotopes are suppressed by calibrating the angles and frequencies of the interferometry beams. We find an Eötvös parameter of η = [1.6 +/- 1.8 (stat) +/- 3.4 (syst)] x 10^-12, consistent with zero violation of the equivalence principle. With a resolution of up to 1.4 x 10^-11 g per shot, we demonstrate a sensitivity to η of 5.4 x 10^-11 / (Hz)^1/2.