Testing gravity with cold-atom interferometers

We present a horizontal gravity gradiometer atom interferometer for precision gravitational tests. The horizontal configuration is superior for maximizing the inertial signal in the atom interferometer from a nearby proof mass. In our device, we have suppressed spurious noise associated with the horizonal configuration to achieve a differential acceleration sensitivity of 4.2x10(-9) g/root Hz over a 70-cm baseline or 3.0x10(-9) g/root Hz inferred per accelerometer. Using the performance of this instrument, we characterize the results of possible future gravitational tests. We demonstrate a statistical uncertainty of 3x10(-4) for a proof-of-concept measurement of the gravitational constant that is competitive with the present limit of 1.2x10(-4) using other techniques. From this measurement, we provide a statistical constraint on a Yukawa-type fifth force at 8x10(-3) near the poorly known length scale of 10 cm. Limits approaching 10(-5) appear feasible. We discuss improvements that can enable uncertainties falling well below 10(-5) for both experiments.