Researchers Report Field Success with Quantum Gravity Gradiometer

Researchers from the UK National Quantum Technology Hub in Sensors and Timing at the University of Birmingham used quantum technology to detect an object hidden below the ground. They said this is the first time a quantum gravity gradiometer has seen practical use outside of laboratory conditions.

The gradiometer was developed under a contract for the Ministry of Defence and in the UK Research and Innovation (UKRI)-funded Gravity Pioneer project. The researchers used the device to find a tunnel in real-world conditions, buried outdoors 1 m below the ground surface.

The quantum gravity gradient sensor in use underground. To enable gravity cartography and operation in application-relevant conditions, researchers implemented a cold atom gravity gradiometer, which enabled robust coupled differential measurements on two clouds of atoms separated by a vertical baseline. In the setup, two counter-oriented single-beam magneto-optical traps allowed the passage of common Raman beams to perform interferometry. Courtesy of the University of Birmingham.

The quantum gravity gradient sensor works by detecting variations in microgravity using the principles of quantum physics, which are based on manipulating nature at the submolecular level. The technology could open a commercial path to significantly improved mapping of conditions below ground level. It could potentially reduce costs and delays to construction, rail, and road projects. It could improve the prediction of natural phenomena such as volcanic eruptions and the discovery of hidden natural resources and underground structures. And it could have implications for archaeology.

“This is an ‘Edison moment’ in sensing that will transform society, human understanding, and economies. With this breakthrough we have the potential to end reliance on poor records and luck as we explore, build, and repair,” said Kai Bongs, head of Cold Atom Physics at the university and principal investigator at UK Quantum Technology Hub Sensors and Timing. “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.”

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 could be addressed, surveys could be faster and more comprehensive, and could cost less.

The gradiometer was developed by a team led by Michael Holynski, head of Atom Interferometry at Birmingham and lead author of the study. The device is able to overcome a variety of challenges, such as microseismic and laser noise, thermal and magnetic field variations, and instrument tilt.

The successful detection, realized in collaboration with civil engineers led by professor Nicole Metje of the university's School of Engineering, is the culmination of a long-term development program that has been closely linked to end users from its outset.

A perspective of future gravity cartography used with 0.5-m spatial resolution over a region, at an uncertainty level of 20 E. Expected signal sizes are shown for a range of applications. Courtesy of the University of Birmingham.

“Detection of ground conditions such as mine workings, tunnels, and unstable ground is fundamental to our ability to design, construct, and maintain housing, industry, and infrastructure. The improved capability that this new technology represents could transform how we map the ground and deliver these projects,” said George Tuckwell, director of geoscience and engineering at RSK Group.

The work is a collaboration between the University of Birmingham; environmental, engineering, and sustainability solutions provider RSK; the Defence Science and Technology Laboratory, part of the U.K. Ministry of Defence; and technology company Teledyne e2v. The project is funded by UKRI as part of the UK National Quantum Technologies Programme, and under contract from the Ministry of Defence.

The research was published in Nature (www.doi.org/10.1038/s41586-021-04315-3).