| An absolute gravity gradiometer was demonstrated using atom interference techniques. This is the first realization of an gradiometer which uses an absolute standard for its calibration. A gravity gradiometer measures spatial changes in the gravitational field over a fixed baseline by making simultaneous acceleration measurements with two spatially separate accelerometers. The gradiometer has a differential sensitivity of 4 × 10−9 g in 1 s and a differential accuracy of 10 −9 g. This is the best gradiometer accuracy reported to date and the sensitivity competes favorably with existing state-of-the-art instruments. A proof-of-principle measurement of the gravity gradient of a small test mass was made leading towards a precision measurement of the gravitational constant. The performance was characterized on a vibrationally noisy reference platform, testing the ability of the gradiometer to reject common-mode accelerations. Techniques for extracting gradient information were explored. Applications of sensitive and accurate gravity gradiometers include tests of general relativity, studies of the gravitational constant, navigation, and geophysical studies.; The principle behind the measurement is as follows: proof masses for the two accelerometers consist of two ensembles of laser-cooled cesium atoms whose acceleration is measured by an interferometer sequence. The interferometer is comprised of light pulses in a π/2 − π − π/2 pulse sequence which acts to divide, deflect, and recombine each atomic wavepacket. The final state of the atom depends on the inertial forces experienced by the atom during its trajectory through the interferometer. The two simultaneous acceleration measurements are subtracted to produce a gravity gradient. This technique is advantageous because it offers intrinsic absolute calibration, robust operation, and uniformity of proof masses. |
