Development of an Atom Interferometer Gravity Gradiometer for Earth Sciences*

We report progress towards a prototype atom interferometer gravity gradiometer for Earth science studies from a satellite in low Earth orbit. The terrestrial prototype has a target sensitivity of $8 \times 10^{-2} \mathrm{~E/Hz^{1/2}}$ and consists of two atom sources running simultaneous interferometers with interrogation time $T = 300 \mathrm{~ms}$ and $12\hbar k$ photon recoils, separated by a baseline of 2 m. By employing Raman sideband cooling and magnetic lensing, we will generate atomic ensembles with $N = 10^6$ atoms at a temperature of 3 nK. The sensitivity extrapolates to $7 \times 10^{-5} \mathrm{~E/Hz^{1/2}}$ in microgravity on board a satellite. Simulations derived from this sensitivity demonstrate a monthly time-variable gravity accuracy of 1 cm equivalent water height at 200 km resolution \footnote{S.B. Lutchke, \textit{et al.} In proceedings of the American Geophysical Union Fall Meeting, San Francisco, California, 2016}, yielding an improvement over GRACE by 1-2 orders of magnitude. A gravity gradiometer with this sensitivity would also benefit future planetary, lunar, and asteroidal missions.