An object hidden below ground has been located using quantum technology - a long-awaited milestone with profound implications for industry, human knowledge and national security.1 The quantum gravity gradiometer, which was developed under a contract for the Ministry of Defence and in the UKRI-funded Gravity Pioneer project, was used to find a tunnel buried outdoors in real-world conditions one metre below the ground surface. It wins an international race to take the technology outside.

The sensor works by detecting variations in microgravity using the principles of quantum physics, which is based on manipulating nature at the sub-molecular level. The success opens a commercial path to significantly improved mapping of what exists below ground level.2

This will mean:

  • Reduced costs and delays to construction, rail and road projects.
  • Improved prediction of natural phenomena such as volcanic eruptions.
  • Discovery of hidden natural resources and built structures.
  • Understanding archaeological mysteries without damaging excavation. 

Professor Kai Bongs, Head of Cold Atom Physics at the University of Birmingham and Principal Investigator of the UK Quantum Technology Hub Sensors and Timing, said: “This is an ‘Edison moment’ in sensing that will transform society, human understanding and economies.3

  • 1. University of Birmingham researchers from the UK National Quantum Technology Hub in Sensors and Timing have reported their achievement in Nature. It is the first in the world for a quantum gravity gradiometer outside of laboratory conditions.
  • 2. Current gravity sensors are limited by a range of environmental factors. A particular challenge is vibration, which limits the measurement time of all gravity sensors for survey applications.  If these limitations can be addressed, surveys can become faster, more comprehensive and lower cost.
  • 3. “With this breakthrough we have the potential to end reliance on poor records and luck as we explore, build and repair. In addition, an underground map of what is currently invisible is now a significant step closer, ending a situation where we know more about Antarctica than what lies a few feet below our streets.”

Stray, B., Lamb, A., Kaushik, A. et al. Quantum sensing for gravity cartography. Nature, 2022 DOI: 10.1038/s41586-021-04315-3

The sensing of gravity has emerged as a tool in geophysics applications such as engineering and climate research, including the monitoring of temporal variations in aquifers and geodesy. 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. Here we overcome this limitation by realizing a practical quantum gravity gradient sensor. Our design suppresses the effects of micro-seismic and laser noise, thermal and magnetic field variations, and instrument tilt. 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. Using a Bayesian inference method, we determine the centre to ±0.19 metres horizontally and the centre depth as (1.89 −0.59/+2.3) metres. The removal of vibrational noise enables improvements in instrument performance to directly translate into reduced measurement time in mapping. The sensor parameters are compatible with applications in mapping aquifers and evaluating impacts on the water table, archaeology, determination of soil properties and water content, and reducing the risk of unforeseen ground conditions in the construction of critical energy, transport and utilities infrastructure, providing a new window into the underground.