Development of a Quantum Gravity Gradiometer for Observatory Measurements

Monitoring the gravitational field at the surface of the Earth is of great interest for Geodesy and for the characterization of underground mass displacements. However, the interpretation of gravity data can be ambiguous because different geophysical sources can lead to similar signature on the gravitational field. Given that the acceleration of the Earth's gravity g and the gravity gradient scale differently with the distance to the mass source of interest, it appears fruitful to measure these two parameters simultaneously to circumvent the problem of geophysical source separation.

We report on the development of an industry-grade gravity gradiometer that is able to measure simultaneously the gravity acceleration and its gradient. This quantum gravity gradiometer relies on atom interferometry with cold 87Rb atoms: it measures the vertical gradient of g with a single vertical laser beam interrogating simultaneously two sets of laser-cooled atoms at different height [REF]. Such device relies on a physical principle and a set of technologies that have already been validated for absolute quantum gravimeters [2]. We present the performance of the device that has been integrated and discuss its stationary measurement capability, as a sensitivity of 60 E/√Hz (1E = 10-9 s-2 = 0.1 µGal/m) and a resolution of 1E are expected. We will also review the operational features of this new device including its transportability and ease of use. This new type of gravity gradiometer is to our knowledge the only demonstrated technology that allows for an absolute continuous drift-free monitoring of both gravity and gravity gradient over timescales from a few minutes to several months.

This work demonstrates the feasibility to operate a free-falling atom gravity gradiometer as a turn-key device and paves the way to practical investigation of both spatial and temporal gravity gradient variations at a level of 1 E in both laboratory and field conditions.

[1] M. J. Snadden et al. "Measurement of the Earth's Gravity Gradient with an Atom Interferometer-Based Gravity Gradiometer" , Phys. Rev. Lett. 81, 971 (1998)

[2] V. Ménoret et al., "Gravity measurements below 10-9 g with a transportable absolute quantum gravimeter", Nature Scientific Reports, vol. 8, 12300 (2018)