Yukun Yang and Minkang Zhou, Huazhong University of Science and Technology, China, compiled by Michael Allen. Physics World , 2021, (12): 44
This article is taken from Physics, Vol. 1, No. 1, 2022
Much of what is known as quantum technology is related to computation, and a good, powerful computer is very attractive for solving complex problems. Computers as data processing tools, both quantum and classical, are used in industry, medicine, and other fields only after converting data into information, but only if there are high-quality sensors used to collect the data necessary for these critical data.
In fact, new sensors and new ways of collecting information have triggered many technological and economic revolutions, "If we look back at history, we will find that the Nobel Prize is always associated with the invention of sensors, such as the 1901 R?ntgen (Wilhelm R?ntgen) for the discovery of X-rays and won the Nobel Prize. ", says Kai Bongs, a quantum physicist from the University of Birmingham in the UK, "Nobody knew what that was at the time, but today X-rays are used in almost every hospital, in airport scanners, in many quality control machines in industry, etc., and they give us information about the inside of an object that we couldn't see before. "
Figure 1 Quantum gravimeters have many potential engineering applications because they are faster and more accurate
Bongs is the academic lead for the UK's Centre for Quantum Sensing and Timing Technologies, one of the UK's national quantum technology programs with more than 110 sub-projects and a total of ￡120 million in funding, which aims to promote innovation in quantum sensors such as magnetic sensors and gravity sensors, among others, for innovation and commercialization. These technologies have applications in many areas, including climate, communications, energy, transportation, healthcare and urban development.
"When we talk about quantum sensors, we mean utilizing quantum effects such as superposition or possibly entanglement," said Bongs. Superposition refers to the ability of a particle to be in two quantum states at the same time or to travel along two paths at the same time. "When these two paths reconverge, the difference between the wave packets between the two paths will create a quantum interference phenomenon at the end, which allows us to measure the physical fields that cause this difference with very high precision." Bongs explains.
This quantum effect was used by Bongs and his colleagues to prepare an atomic interference-based quantum gravity gradiometer, in which two clusters of atoms are at different heights and evolve along two different paths, meaning that the two clusters will feel the gravitational field with a very small difference at the different heights, and the phases of the interference fringes will contain both gravity and gradient information.
Most classical gravimeters can be equated to suspending a spring with a mass, and reflecting changes in gravity by measuring the spring's expansion and contraction. The disadvantage is that the spring itself is stretched by ground vibrations, which means that such instruments need to be constantly calibrated before they can be used, and that each reading needs to be waited for a sufficiently long period of time to average out the effects of the background noise introduced by ground vibrations, which include passing trucks and trains, and the background noise introduced by the ground vibrations. include passing trucks, trains, and low-intensity seismic activity, as well as other vibrations.
Although spring-loaded gravimeters are very sensitive, quantum gravimeters have an advantage: regardless of how the ground is vibrating, a quantum gravimeter has only one overall pattern of motion, with no spring-like elasticity characteristic. The Quantum Gravimeter's device, the cluster of atoms, and the laser that detects the fall of the atoms move together. "You can eliminate unnecessary sources of sensitivity," says Bongs, "and at the same time you can suppress noise such as ground vibrations and increase sensitivity." He added.
In civil engineering, gravity sensors are used to detect differences in mass distribution underground to help find buried infrastructure such as pipelines, tunnels, and old mines. Other technologies such as ground-penetrating radar, while also used for this work, differ in that they are active technologies that must transmit a signal into the ground, with their detection distance limited by the attenuation of the signal's propagation. "The real advantage of gravity is that it's passive, we don't have to pre-input the signal, we just have to measure it on the ground." Daniel Boddice, a civil engineer from Birmingham, explains. As long as the underground material generates a large enough gravitational signal at the surface, we can detect it.
Bongs admits that despite the potential of gravity sensors, they are not commonly used even in geophysics, mainly for this reason: in order to cancel out the effects of vibrational noise, you have to probe a location long enough to accumulate a large amount of data, which is therefore expensive. Nicole Metje, a civil engineer from Birmingham, believes that it's not just seismic vibrations that generate noise signals, "When you're in environments like public **** traffic, people walking around, drilling operations, those all generate vibrations." Boddice adds, "The real advantage of quantum sensors is that we can use it in more places and measure faster, more efficiently, and more accurately."
Recently Metje and Boddice used a quantum gravimeter to detect culverts (pipes or structures that act as drainage under railroad tracks) on railroad tracks. If they are blocked, the roadbed becomes saturated with water, creating what is known as a "wet bed," which affects the stability of the track and creates structural problems similar to camber, which can affect the safe operation of trains and cause delays. These culverts may be buried deep beneath the track, making it difficult to locate them and assess their condition. Since the depth of detection of ground-penetrating radar is sometimes insufficient and engineers usually have only a few hours at night to make measurements, Metje considered gravity sensors to be more effective than any other method in such cases. However existing spring-based gravity sensors measure slowly, but quantum gravity sensors measure faster because they don't have the noise generated by vibrations, and they don't need to be stationary. This quantum sensor could therefore be mounted on a train and scan the tracks while the train is moving. The Birmingham group has already tested it on some railroad tracks in the UK.
George Tuckwell from the group at RSK, a UK-based environmental and engineering consultancy, looked at how quantum gravity sensors could be used on civil engineering projects, and RSK has helped clients in the early days to mitigate the risks of construction projects by assessing the condition of the ground, and they have mapped the ground to identify changes in bedrock and groundwater, as well as other natural and man-made changes in the ground. changes, such as landfills and mining operations. This avoids the impact on the project of unforeseen risks that could bring about lost funding and delays.
Quantum gravity can also be used to enhance the effectiveness of navigation systems, and in recent years increasing attention has been paid to carrier position deviations caused by GPS navigation errors. Particularly in the field of marine navigation, when a ship receives an erroneous navigation signal, the ship will incorrectly estimate the true position it is in, and hostile forces or pirates are likely to take advantage of this error to hijack and sabotage the ship, or even to guide it to the range of hostile forces' waters - recreating a 21st-century version of the Cornish shipwreck accident with a guiding lantern.
If we could map an accurate gravity grid, ships could use the quantum gravimeters they carry to record gravity values and compare them to the grid to determine their position. In theory, the gravimeter would be able to be completely sealed in a box, isolated from the outside world, which makes it impervious to break-ins. Even if someone cuts off the ship's communications, satellite and radar navigation systems or even anything that can connect to the outside world, the gravity will still be able to navigate. "The only way to significantly interfere with the gravity sensor is to change the gravity signal, and that means moving a mass the size of a mountain." Bongs explained.
Indeed, gravimeters can be used to "detect invisible signals," said Dr. Bruno Desruelle, chief executive officer of France's μQUANS. The company has been using absolute quantum gravimetry to study geological activity near the summit of Italy's Etna volcano for a year, and researchers will soon be able to obtain and publish new information on the Etna volcano.
Figure 2 Quantum absolute gravimeter used to study volcano properties
Measuring changes in gravity near a volcano is important because it will give us changes in the density of subsurface materials such as rocks, gases, and magma. An increase in gravity is likely to mean an influx of dense material such as magma, while a decrease in density (a decrease in gravity) implies the presence of seepage pits. "The idea of this type of study is to use measurements of gravity at the surface of a volcano to invert geophysical processes in the subsurface to gain a deeper understanding of the movements inside the volcano." Desruelle explained.
Quantum gravimetry has reached the practical stage, Desruelle said, citing examples of their specific applications. "As long as you want to know the distribution of mass in the subsurface, you will use quantum gravity sensors in a variety of activities, and this includes hydrology, seismology, and civil engineering projects to detect crevices, seepage pits, tunnels, and cavities. A lot of people are interested in instruments for geodesy," he added, "so they want to learn more about the geosphere and gravity maps, and a lot of research organizations are assigned to gravity nets in different regions."
For engineering and geophysical applications, the two different paths of an atom's descent are separated by just a few millimeters, but simply increasing the size of the instrument can dramatically increase the sensitivity, and can be used to detect the unknown, invisible matter that makes up more than 85% of the matter in the universe (dark matter, dark energy).
As of January this year, the UK's Research and Innovation Foundation funded seven projects worth up to ￡31 million, hoping to use quantum technology to solve major problems in fundamental physics. Three of these are developing quantum-enhanced interferometers and sensors to search for dark matter - detecting candidates such as axions or testing quantized theories of space-time, for example. Often disruptive discoveries in science are made on the basis of the convergence of new technologies and established theories, and we may still be at the dawn of quantum sensing, but it is fascinating and desirable that it is already showing us a better path and leading us to explore the deepest mysteries of the universe.