Quantum physics: Quantum sensors open a window to the underground

An atom-based sensor detects features beneath the Earth’s surface by precisely measuring changes in the gravitational field, a study in Nature demonstrates. The work supports the notion that quantum sensing can provide a new tool for inspecting the underground, which could help map out geological structures or measure archaeological artefacts.

Sensors that detect variations in gravitational fields have the potential to map out large-scale structures on or below Earth’s surface. For example, electromechanical gravimeters have been used for geophysical surveys. However, employing this technique at smaller scales (around the metre-scale), for instance in building sites, is challenging as a long measuring time is needed to cancel out local seismic noise or vibrations, such as from moving vehicles or other structures.

These limitations can be overcome if the effects of noise are suppressed by measuring differences in gravity instead of absolute values, Michael Holynski and colleagues explain. This can be implemented using quantum systems, which are highly sensitive to their surroundings and thus make ideal sensing tools. The authors’ design is realized in an atom-based device, the sensitivity of which is demonstrated by performing a survey in an urban environment in the city of Birmingham, UK. The instrument detects a 2 metre by 2 metre tunnel under a road surface between two multi-storey buildings.

This sensor may have potential for a range of applications, the authors suggest. Civil engineers might be able use it to inspect brownfield sites for near surface (10m below the ground surface) features that could affect new constructions. Archaeologists may find it useful for mapping out tombs and structures hidden beneath the ground. It could even find use in the measurement of geological features, such as aquifers or soil density to determine water content.