Quantum Sensor Opens ‘a New Window into the Underground’

A team of physicists and engineers have road-tested a quantum sensor that can probe underground structures by harnessing the mind-boggling physics that governs atoms and gravity, according to a new study. 

This type of sensor, known as a quantum gravity gradiometer, has been successfully tested before in laboratory conditions, but the new research reports the first demonstration of the instrument in a practical outdoor setting, a breakthrough that paves the way toward a host of improved sensing applications in fields as diverse as archeology, navigation, urban planning, and disaster preparation.

Researchers led by Michael Holynski, a physicist and senior lecturer at the University of Birmingham’s Cold Atoms research group, developed a novel gradiometer that sensed a tunnel buried three feet beneath a road in Birmingham, which is exactly the sort of noisy vibrational environment that has scuttled the practical performance of quantum sensors in the past. 

The success of the milestone detection opens up “a new window into the underground” and lays the foundations for “faster mapping or detection of smaller and deeper features” in subterranean locations, reports the team in a study published on Wednesday in Nature. Given that major archeological finds have been unearthed from under car parking lots and urban infrastructure is often too complex for traditional sensing methods, the new achievement could offer a more efficient and effective way to examine inaccessible spaces.

“It was really exciting for the team because it's been a culmination of many years of effort to get to a situation where we can even use the sensor outside,” said Nicole Metje, a professor of infrastructure monitoring at the University of Birmingham who co-authored the study, in a joint call with Holynski. 

“We knew that it was something quite special,” Metje added. “It's a joint effort between physics and engineering, which really came through.”

Researchers have developed a range of tools to map out underground spaces without physically digging into them, but gravity sensors have the potential to peer even deeper into Earth and assess a wider range of environments, such as ancient tombs, mine shafts, or water aquifers. These sensors manipulate the quantum properties of atoms to measure minute variations in gravitational fields, a technique that can reveal the contours and properties of subterranean areas in urban areas. 

However, these instruments are so sensitive that they pick up all the messy vibrations that shake real-world environments, such as the wind blowing, people walking, or construction projects. These vibrations interfere with the gravitational signals that the sensor relies on to see into hidden spaces, necessitating long data-collection and processing times as researchers try to extract the right signal from the noise. 

“Vibration is relevant for all gravity sensors,” explained Holynski. “It doesn't matter how sensitive they are, or how they're made—we can't distinguish between gravity and vibration. There's no way of doing that because of Einstein's equivalence principle. It's just a fundamental law of physics.”

To overcome this problem, Holynski, Metje, and their colleagues at the UK National Quantum Technology Hub in Sensors and Metrology, developed a gradiometer that leverages a trippy quantum phenomenon known as superposition in which an atom can occupy two states at once. 

The team’s instrument fires a laser pulse at clouds of cold atoms separated vertically by three feet. This initial trigger prompts some of them to enter a state of superposition where one version of the atom has absorbed the pulse, generating momentum, and one has not. A second pulse causes the atoms to switch these conditions so that the atom that initially avoided the pulse now absorbs it, and vice versa, and a third pulse brings them back to the original state. 

This method, known as atom interferometry, allows the researchers to measure the difference between the trajectories of the atoms, which reveals minute variations in the gravitational field of masses below them. Because the two clouds of atoms are excited by the laser pulses at the exact same time, they encounter the same vibrational noise but record different gravitational signals because they are at different elevations, which enables researchers to isolate the gravitational data from the vibrational feedback

“When we take the difference of those measurements, the vibration is just removed and the gravity signal is kept,” Holynski said. “The brilliance of that is that if I take a gravity sensor, even if I make it ten or a hundred or a thousand times more sensitive, it's still limited by the vibrations, and I still have to wait for that to be averaged away.”

“With the sensor that we've made now, if we make the instrument 10 or a hundred thousand times more sensitive, we can actually measure faster and see things quicker,” he continued. “And that's how we have overcome this vibration challenge.”

Now that they have successfully demonstrated this approach outdoors, Holynski and his colleagues are focused on miniaturizing their gradiometer, which is currently about the size of a washing machine. The team aims to eventually develop instruments that are the size of backpacks or even soda cans, making them more practical for scientific and industrial uses. The researchers are also exploring ways to map underground spaces while the sensor is in motion, which could enable precise and large-scale surveys across a wide range of landscapes.

In the future, these sensors could be a kind of “Google Maps of the underground,” Holynski said. They could be used to peer into delicate archeological sites, predict floods by probing underground reservoirs, detect sinkholes before they cause infrastructure damage, examine soil properties for agricultural research, among many other applications.  

“At the moment, gravity sensors are not the sensor of choice—it is one of the sensors you use if everything else fails, almost,” Metje said. “But what this now enables is that it is starting to become more of a sensor of choice, simply because we can do things much faster, because we don't have to spend so much time post-processing to get rid of all the noise.”

“Rather than being, from an engineering viewpoint, a sensing technology on the sideline, it hopefully becomes much more mainstream,” she concluded.