Measurement of the Earth’s Gravity Gradient with an Atom Interferometer-Based Gravity Gradiometer

Measurement of the gradient of gravitational fields has important scientific and technical applications. These applications range from measurement of G, the gravitational constant, and tests of general relativity [1,2] to covert navigation, underground structure detection, oil-well logging, and geodesy [3]. This Letter describes the development of a gravity gradiometer whose operation is based on recently developed atom interference and laser manipulation techniques. A crucial aspect of our design is its intrinsic immunity to spurious accelerations. Our method is illustrated in Fig. 1. We use light pulse atom interferometer techniques [4–6] to measure the simultaneous acceleration of two laser cooled ensembles of atoms. The relative acceleration of the atom clouds is measured by driving Doppler-sensitive stimulated twophoton Raman transitions [7] between atomic groundstate hyperfine levels. Our geometry is chosen so that the measurement axis passes through both laser cooled ensembles. Since the acceleration measurements are made simultaneously at both positions, many systematic measurement errors, including platform vibration, cancel as a common mode. This instrument is fundamentally different from current state-of-the-art instruments [8,9]. First, the proof masses used in our work are individual atoms rather than precisely machined macroscopic objects. This reduces systematic effects associated with the material properties of macroscopic objects. Second, the calibration for the two accelerometers is referenced to the wavelength of a single pair of frequency-stabilized laser beams, and is identical for both accelerometers. This provides long term accuracy. Finally, large separations s¿1 md between accelerometers are possible. This allows for development of high sensitivity instruments. Our design also differs significantly from that of previously proposed atom interference-based instruments [10]. The “figure-eight” geometry of prior proposals essentially implements two sequential acceleration measurements with a single ensemble of atoms. Consequently, it exhibits poor immunity to platform vibrations. In contrast, our instrument is based on simultaneous acceleration measurements.