New Quantum Sensor Sees Beneath the Beneath

Subterranean scanners promise applications in construction, mining

5 min read

Photo of researchers in an underground geological formation, standing underneath portable tarp with boxes full of equipment

Researchers from the University of Birmingham, in England, have tested a portable quantum sensor [see image below] as an ultraprecise gravity-measuring device—testing it in such locations as the subterranean caves of Poole's Cavern, a geological attraction near Birmingham.

University of Birmingham

Just as quantum computers can theoretically find the answers to problems no classical computer could ever solve, so too can quantum sensors lead to new levels of sensitivity and accuracy. Now scientists reveal their quantum-gravity sensor (a sensor of gravity involving ultraprecise quantum technologies—and not, to be clear, a sensor that harnesses quantum gravity) can help map features hidden underground with unprecedented detail, a new study finds.

Quantum technology relies on quantum effects that can emerge due to how the universe becomes a fuzzy place at its very smallest levels. For instance, the quantum effect known as superposition allows atoms and other building blocks of the cosmos to essentially spin in two opposite directions at once or exist in two or more places at the same time. By placing many components known as qubits into superposition, a quantum computer can in theory perform a mind-boggling number of computations simultaneously.

Of course quantum sensors are, like quantum computers, notoriously very fragile to outside interference. Yet, quantum sensors also capitalize on this vulnerability to achieve exceptional sensitivity to the least disturbances to the environment, with many potential applications, such as medicine, nanotechnology, telecommunications, and satellite navigation.

The gravity sensor can detect a 2-by-2-meter utility tunnel 0.5 meters underground—and can collect 10 data points in 15 minutes.

For instance, in November, scientists in England and Germany revealed their magnetic sensor could help noninvasively detect magnetic changes in brain activity that result when neurons fire. The device contains a gas of rubidium atoms illuminated by lasers, and when these atoms experience changes in a magnetic field, they emit light differently. The quantum sensor can prove far more accurate than either EEG or fMRI scanners, with temporal and spatial resolutions down to milliseconds and several millimeters, and is now commercially available via the British startup Cerca Magnetics.

Another promising kind of quantum sensor uses defects in diamonds. Perfect diamonds are made of pure carbon, but sometimes a nitrogen atom can sneak in, creating a defect. Such “nitrogen vacancy defects” hold electrons that can absorb green light and emit red photons when near a very weak magnetic field. Scientists can use this feature to help create 3D images of molecules to better analyze, say, potential medicines.

Now scientists in England have developed a new quantum sensor for gravity mapping that they say is capable of unmatched subterranean scanning outside the lab. They detailed their findings in a recent issue of the journal Nature.

Anything that has mass has a gravitational field that attracts objects toward it. The strength of this field depends on a body’s mass. Since Earth’s mass is not spread out perfectly evenly, this means the planet’s gravity is stronger at some places and weaker in others.

For decades, researchers have analyzed variations in the strength of Earth’s gravitational field to map large-scale geological activity, such as magma churning under Earth’s surface, the melting of glaciers, or the way major earthquakes can deform the planet. However, employing such gravity cartography on the scale of meters is challenging, since long measuring times are needed to account for local noise, such as vibrations from nearby traffic.

“Ours is the first to really work outside and still be sensitive enough to find tunnels.”
—Kai Bongs, University of Birmingham

The group’s new gravity sensor uses clouds of about 100 million rubidium atoms cooled to two- or three-millionths of a degree Celsius above absolute zero. It analyzes the rate at which a cloud falls to deduce the local strength of Earth’s gravitational pull.

Picture of metallic shielding surrounding thick glass window with a laser apparatus insideThis laser atom trap containing a supercooled cloud of rubidium atoms performs ultraprecise measurements of their gravitational displacements, forming the foundation of the ground-penetrating sensing that the device performs. University of Birmingham

Specifically, laser pulses drive the atoms into a state of superposition, with two versions of the atoms falling down slightly different trajectories. Those Schrödinger’s cat–like states of the atoms are then recombined. Then, due to wave-particle duality—the quantum phenomenon where particles can act like waves, and vice versa—these atoms quantum mechanically interfere with each other, with their peaks and troughs augmenting or suppressing each other. Analyzing the nature of this interference can reveal the extent of the slightly different gravitational pulls felt on their separate paths.

The sensor uses an hourglass configuration, with one cloud in each half of the device separated vertically by 1 meter. As such, the sensor can measure the strength of Earth’s gravity at two different heights at the same location. By comparing the data from these clouds, the researchers can account for a variety of sources of noise, such as vibrations, thermal and magnetic-field variations, randomness in the lasers, and tilting of the sensor.

In experiments, the sensor could detect a 2-by-2-meter utility tunnel buried roughly 0.5 meters under a road surface between two multistory buildings in the city of Birmingham, England. It could collect 10 data points in 15 minutes.

“The hourglass configuration developed in our team has allowed a step change in robustness, which ultimately has led to these pioneering measurements,” says study co–senior author Kai Bongs, a quantum physicist at the University of Birmingham.

outdoor photo of gear, cabling and wheeled boxResearchers say they‘re now developing a backpack-size version of their gravity sensor [pictured here].University of Birmingham/Crown Copyright

This new device is about one-thirtieth as sensitive as the best quantum sensor of gravity that’s been reported yet, says Nicola Poli, an experimental physicist at the University of Florence, in a commentary on the new study. However, Poli, who did not take part in the new work, notes the virtue of the new device is that it can actually find use outside the lab in real-world conditions.

“This kind of quantum technology has been proven in laboratories around the world before, but these instruments were not suitable to run outside, being prone to environmental effects,” Bongs says. “Ours is the first to really work outside and still be sensitive enough to find tunnels.”

There are many potential applications for this sensor, Bongs says. In civil engineering, it can see hidden underground structures such as tunnels, mine shafts, and sinkholes to reduce construction risks. In mining, it can help discover subterranean natural resources. In archaeology, it can discover underground mysteries without damaging excavation. It can also help monitor magma flows under volcanoes to warn of potential eruptions and groundwater levels and flows to help improve flooding models.

Bongs noted the new sensor is not capable of creating detailed images “of, for example, people in houses. The resolution is too low for this.”

The scientists are now developing a backpack-size version of their quantum gravity sensor for use on mobile platforms. The researchers envision their device will initially be used in a stop-and-go fashion, pausing to measure Earth’s gravity at one spot and then moving to a neighboring spot. Further advances may allow gravity cartography with the sensor “on a continuously moving platform,” Bongs says.

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