The measurement axis is aligned to measure the vertical component, G zz, of the (3 × 3) gravity gradient tensor, which is the largest and most relevant component for gravity cartography. Two counter-oriented single-beam magneto-optical traps (MOTs) allow passage of common Raman beams to perform interferometry (Fig. This enables robust coupled differential measurements on two clouds of atoms, separated by a vertical baseline 28. To enable gravity cartography, and operation in application-relevant conditions, we implement an ‘hourglass’ configuration cold atom gravity gradiometer 27. This presents a major barrier to realizing gravity maps with high spatial resolution. However, such devices, as with any gravimeter, are fundamentally limited in their measurement time owing to the need to average out micro-seismic vibration 26. The phase difference in the resulting interference pattern is proportional to the local gravitational field. This creates the matter-wave analogue of a Mach–Zehnder interferometer. The resulting atomic wavepackets move along two spatially separated trajectories, before being recombined and interfered. A typical approach in these devices is to use light pulses to drive two-photon stimulated Raman transitions in atoms and use these to create a superposition of matter waves in different momentum and energy states. For example, gravity sensors have been created that can be used on volcanoes and mountain environments 21, 22, and for measurements by air 23, by sea 24 and on rockets 25. The quantum gravity gradient sensor uses atom interferometry 15, which has been used in laboratory-based experiments to provide sensitive measurements of gravity 16, to investigate the equivalence principle 17, the fine-structure constant 18 and Newton’s gravitational constant 19, prompting the desire to transition these sensors into practical devices for use in real-world environments 20. The sensor parameters are compatible with applications in mapping aquifers and evaluating impacts on the water table 7, archaeology 8, 9, 10, 11, determination of soil properties 12 and water content 13, and reducing the risk of unforeseen ground conditions in the construction of critical energy, transport and utilities infrastructure 14, providing a new window into the underground. The removal of vibrational noise enables improvements in instrument performance to directly translate into reduced measurement time in mapping. Using a Bayesian inference method, we determine the centre to ☐.19 metres horizontally and the centre depth as (1.89 −0.59/+2.3) metres. The instrument achieves a statistical uncertainty of 20 E (1 E = 10 −9 s −2) and is used to perform a 0.5-metre-spatial-resolution survey across an 8.5-metre-long line, detecting a 2-metre tunnel with a signal-to-noise ratio of 8. Our design suppresses the effects of micro-seismic and laser noise, thermal and magnetic field variations, and instrument tilt. Here we overcome this limitation by realizing a practical quantum gravity gradient sensor. However, it is impractical to use gravity cartography to resolve metre-scale underground features because of the long measurement times needed for the removal of vibrational noise 6. The sensing of gravity has emerged as a tool in geophysics applications such as engineering and climate research 1, 2, 3, including the monitoring of temporal variations in aquifers 4 and geodesy 5. Nature volume 602, pages 590–594 ( 2022) Cite this article
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