Birmingham University researchers from the UK National Quantum Technology Hub in Sensors and Timing have reported their achievement in Nature.
The quantum gravity gradiometer, developed under contract for the Ministry of Defence and in the UKRI-funded Gravity Pioneer project, was used to find a tunnel buried outdoors one metre below the ground surface.
In use, the quantum gravity sensor measures subtle changes in the pulling strength of gravitational fields when a cloud of atoms is dropped.
Professor Kai Bongs, head of Cold Atom Physics at Birmingham University and Principal Investigator of the UK Quantum Technology Hub Sensors and Timing, explained that atoms are put into a quantum superposition of traveling along two different trajectories simultaneously and the respective matter wave phases are allowed to interfere at the end.
This gives rise to a phase-depended imbalance between the number of atoms in one quantum state and the number of atoms in another quantum state.
“The phase difference depends on gravity in this case, so counting the number of atoms in each state at the end provides a means to measure gravity very precisely,” he said via email. “The instrument does this with two atom clouds at different heights, in order to extract the gravity gradient.”
He continued: “In the end we plot the measurement outputs of the lower versus the upper chamber in one graph, resulting in an ellipse shape from which we can extract the gravity gradient at the measurement point. This is done for many points to create a map.”
The sensor, developed by Dr Michael Holynski, Head of Atom Interferometry at Birmingham and lead author of the study, overcomes limitations including vibration, instrument tilt and disruption from magnetic and thermal fields to apply quantum technology in the field.
The successful detection of the tunnel, realised 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.
“We have now created a start-up called Delta-g. We expect a backpack sized prototype within the next two years,” said Prof Bongs.
The sensor could one day be deployed in space, said George Tuckwell, director of Geosciences & Engineering and Innovation Lead - RSK Group, which led the research.
"The commercial implications of significantly improved mapping of what exists below ground level are huge, particularly for the construction industry which is likely to see reduced costs and delays to construction, rail and road projects,” he said. “The quantum technology, which could ultimately be placed on a satellite to map the Earth from space, also offers improved prediction of natural phenomena such as volcanic eruptions, and allows for underwater and subterranean exploration, including the discovery of natural resources and archaeological mysteries."
Dr Holynski added that Birmingham has a prototype that has a total weight of 15kg and could in principle fit on a small satellite.
“There is still work to do on robustness-for-space,” he said. “There is a significant international effort in cold atoms for space, which has significant implications for fundamental physics, and the UK community is making strides in realising compact payloads that could be used in future Earth observation applications.
The quantum gravity gradiometer breakthrough was made in a collaboration between Birmingham University, RSK, Dstl, and Teledyne e2v.
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