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The Strange Quantum Behavior Of Atoms Was Used By A Novel Gravity Sensor To Peek Beneath

A new gravity sensor used atoms’ weird quantum behavior to peer underground

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The most effective method of locating hidden wealth may be via a quantum gravity sensor. In these devices, free-falling atoms show minor changes in the Earth’s gravitational pull at various locations on the planet. It is believed that these changes represent variances in the density of the material under the sensor, allowing the device to see underground efficiently. According to a recent study published in the journal Nature on February 24, one of these robots successfully detected the minute gravitational signature of an underground tube.

A commentary on the findings appears in the same issue of Nature as the study itself. “Instruments like these would find many, many uses,” says Nicola Poli, an experimental physicist at the University of Florence who co-authored the commentary.
Poli envisions using quantum gravity sensors to monitor groundwater or magma under volcanoes, which might aid archaeologists in discovering buried tombs or other artifacts without the need to dig them up (SN: 11/2/17). Aside from helping farmers assess soil quality, these sensors might also assist engineers in inspecting possible development sites for unstable ground.

It is possible to detect gravity using various methods, according to Xuejian Wu, an atomic physicist at Rutgers University, Newark (N.J.), who was not engaged in the work. Some gadgets measure the distance that gravity drags a mass hanging from a spring to the ground. Other instruments employ lasers to measure the rate at which an item tumbles down a vacuum chamber. However, according to Wu, free-falling atoms, such as those used in quantum gravity sensors, are the cleanest and trustworthy test masses available. Therefore, quantum sensors can be more precise and reliable in the long term than other types of gravitational probes.

The quantum gravity sensor dumps down a chute, which comprises a cloud of supercooled atoms. When a pulse of light is delivered, it separates the falling particles, creating a quantum limbo in which each bit simultaneously lives in two locations (SN: 11/7/19). Because their relative areas in the Earth’s gravitational field are somewhat different, the two copies of each atom experience a little extra downward force as they descend. After that, another light pulse is used to recombine the separated atoms.

By quantum physics’ peculiar wave-particle duality rule (which states that atoms may behave like waves), the re-assembled bits interfere with one another (SN: 1/13/22). This means that whenever two or more atom waves overlap, their crests and troughs can either reinforce or cancel each other out, resulting in an interference pattern. Because of the slightly different downward pulls that the split versions of each atom experienced as they fell, the way revealed the gravity field at the location of the atom cloud.

Using atom-based devices, researchers were able to test Einstein’s theory of gravity (SN: 10/28/20) and measure fundamental constants such as Newton’s gravitational constant (SN: 4/12/18) with unprecedented precision. On the other hand, atom-based gravity sensors are susceptible to vibrations caused by earthquakes, traffic, and other sources.
Physicist Michael Holynski of the University of Birmingham in England says that even very, very small vibrations generate enough noise that it is necessary to measure for a long time at any location to distinguish them from background tremors. As a result, quantum gravity sensing is impractical for many applications outside of the laboratory.
A gravity sensor made of not one, but Holynski’s team developed two falling clouds of rubidium atoms to solve the problem. It was possible to measure the strength of gravity at two different heights in one location by suspending one cloud a meter above another and measuring the difference between them. The researchers were able to cancel out the effects of background noise by comparing the results of their measurements.

Using their sensor, a 2-meter-tall funnel on wheels linked to a moving cart of equipment, Holynski and colleagues attempted to discover an underground corridor on the University of Birmingham campus. They were unsuccessful. The concrete tunnel, measuring two by 2 meters, ran under a road that connected two multistory buildings. Approximately every 0.5 meters along an 8.5-meter line that traversed the tunnel, the quantum sensor monitored the gravity field in the vicinity. A computer simulation had predicted the gravitational signal from the tunnel based on its construction and other elements that may have an impact on the surrounding gravitational field, such as adjacent structures. Those readouts confirmed the expectations of the computer calculation.

According to the researchers, based on the machine’s sensitivity in this trial, it could most likely deliver a credible gravity reading at each place in less than two minutes. Compared to other gravity sensors, this takes just about a tenth of the time.
Since then, the team has developed a smaller version of the gravity sensor utilized in the tunnel-detecting experiment. Compared to the 300-kilogram beast employed for the tunnel test, the new machine weighs approximately 15 kilos. Other upgrades may also increase the speed of the gravity sensor.

Engineer Nicole Metje envisions the development of a quantum gravity sensor that could be moved from one location to another like a lawnmower shortly. However, according to Metje, a co-author of the study affiliated with the University of Birmingham, portability isn’t the only issue to be addressed to make these tools more user-friendly. We still need a physics degree holder to operate the sensor, for the time being, says the researcher.
So hopeful beachcombers may be waiting a long time to trade in their metal detectors for quantum gravity sensors.

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