Compile the | Yukun Yang, Minkang Zhou (Huazhong University of Science and Technology)
Most of the well-known quantum technologies are related to computing, and a good, powerful computer is very attractive at solving complex problems. Computers as data processing tools, whether quantum or classical, are used in industry, medicine and other fields after converting data into information, but only if there are high-quality sensors necessary to collect these critical data.
In fact, new sensors and new ways of collecting information have sparked many technological and economic revolutions, "if we look back at history, we will find that the Nobel Prize has always been related to the invention of the sensor, such as Wilhelm R ntgen won the Nobel Prize for the discovery of X-rays in 1901", said Kai Bongs, a quantum physicist from the University of Birmingham in the United Kingdom. Airport scanners and many industrial quality control machines use X-rays, which allow us to see information inside objects that we couldn't see before. ”
Figure 1 Quantum gravity meters have many potential engineering applications due to their faster and more accurate properties
Bongs is an academic leader at the UK's Centre for Quantum Sensing and Timing Technology, one of the UK's National Quantum Technologies Project, with over 110 sub-projects with a total funding of £120 million, which aims to promote innovation and commercialisation in areas such as quantum sensors such as magnetic sensors and gravity sensors. These technologies have applications in many ways, including climate, communications, energy, transportation, healthcare, and urban development.
"When we talk about quantum sensors, we're referring to the use of quantum effects like superposition or possible entanglement," Bongs said. Superposition refers to the ability of particles to be in two quantum states at the same time or to travel along two paths at the same time. "When the two paths re-converge, the difference between the two paths in the wave packet will produce quantum interference at the end, which allows us to measure the physics that caused this difference with very high accuracy." Bongs explains.
This quantum effect was used by Bongs and his colleagues to prepare atomic interference-based quantum gravity gradients, in which two clusters of atoms are at different heights and evolve along two different paths, meaning that the two clusters of atoms will feel a gravitational field that differs very little at different heights, and the phase of the interference fringes will contain information about gravity and gradient.
Most classical gravimeters can be equivalent to suspending a mass spring that reflects changes in gravity by measuring the expansion and contraction of the spring, with the disadvantage that the spring itself is also stretched by ground vibrations, which means that such instruments need to be constantly calibrated to be used, and each reading needs to wait long enough to average the effects of background noise from ground vibrations, including passing trucks, trains, and low-intensity seismic activity, among other vibrations.
Although the spring-loaded gravimeter is very sensitive, the quantum gravimeter still has advantages: regardless of how the ground vibrates, the quantum gravimeter has only an overall mode of motion, without the elastic characteristics of a spring. The device of the quantum gravity meter, the atomic mass, and the laser that detects the falling of the atoms move together. "You can eliminate unnecessary sources of sensitivity," Bongs says, "and you can also suppress noise such as ground vibrations and improve sensitivity." He added.
In civil engineering, gravity sensors are used to detect differences in the distribution of mass underground to help find buried infrastructure such as pipelines, tunnels, and old mine shafts. Other technologies, such as ground-penetrating radar, are also used for this work, but the difference is that they are active technologies that must transmit signals to the ground, and their detection distance is 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-enter signals, we just have to measure it on the ground." Daniel Boddice, a civil engineer from Birmingham, explains. As long as subsurface matter produces a large enough gravitational signal on the surface, we can detect it.
Bongs acknowledges that despite their great potential, gravity sensors are not even commonly used in geophysics, mainly because in order to eliminate the effects of vibrational noise, you have to probe in one location long enough to accumulate a lot of data, so it is expensive. Nicole Metje, a civil engineer from Birmingham, believes that it's not just seismic vibrations that generate noise signals, "when you're in an environment like public transport, people walking around, drilling operations, etc., those all produce vibrations." Boddice adds, "The real advantage of quantum sensors is that we can use them in more places and measure faster, more efficiently, and more accurately." ”
More recently, Metje and Boddice have used quantum gravimeters to detect culverts (pipes or structures that act as drainage under the rails) on rails. If it is blocked, the road bed will be soaked with water, creating a so-called "wet bed", which will affect the stability of the track and produce structural problems similar to the inclination angle, which will affect the speed of the safe operation of the train, resulting in delays. These culverts can be buried deep under orbit, making it difficult to determine their location and assess their condition. Because the depth of detection of ground-penetrating radar is sometimes not sufficient, and engineers typically only have a few hours to measure at night, in this case, Metje believes that gravity sensors are more effective than any other method. However, existing spring-based gravity sensors are slow to measure, but quantum gravity sensors are faster because there is no noise from vibration, and they do not need to be stationary. So this quantum sensor can be installed on a train and scan the tracks as the train travels. The Birmingham team has already tested it on some of the UK's rails.
George Tuckwell, from a group at RSK, a BRITISH environmental and engineering consultancy, studied how to use quantum gravimeters for civil engineering, and RSK helped clients reduce the risk of construction projects early on by assessing the state of the ground, mapping the ground to identify changes in bedrock and groundwater, as well as other natural and artificial changes on the ground, such as landfills and mining operations. This avoids the impact on the project of unforeseen risks that could lead to loss of funds and delays.
Quantum gravimeters can also be used to enhance the effectiveness of navigation systems, and in recent years there has been an increasing focus on carrier position deviations caused by GPS navigation errors. Especially in the field of maritime navigation, when a ship receives a wrong navigation signal, the ship misestimates its true position, and the hostile forces or pirates are likely to use this mistake to hijack and destroy the ship, or even guide the ship to the sea area of the hostile forces - a guide lantern that reproduces the 21st century version of the Cornish shipwreck.
If we can map an accurate gravity net, the ship can use the quantum gravimeter it carries to record the gravity value and compare it with the gravity net to determine its own position. Theoretically, the gravimeter is able to be completely sealed in a box, isolated from the outside world, which makes it not illegally invaded. Even if someone cuts off the ship's communications, satellite and radar navigation systems, and even all the tools that can connect to the outside world, gravity can still navigate. "The only way to significantly intervene in a gravity sensor is to change the gravity signal, which means moving a mass the size of a mountain." Bongs explains.
Indeed, gravimeters can be used to "detect invisible signals," says Dr Bruno Desruelle, chief consul of France's μQUANS. The company has been using an absolute quantum gravity meter to study geological activity near the summit of Italy's Etna volcano for 1 year, and researchers will soon be able to get and release new information about Etna volcano.
Figure 2 A quantum absolute gravity meter is used to study volcanic properties
Measuring gravitational changes near volcanoes is important because it will give us changes in the density of subsurface material such as rocks, gases, and magma. An increase in gravity is likely to mean the influx of dense material such as magma, while a decrease in density (a decrease in gravity) means the presence of seepage pits. "The idea of this type of study is to use measurements of gravity on the surface of volcanoes to invert geophysical processes underground to gain a deeper understanding of the internal motion of volcanoes." Desruelle explains.
Quantum gravity meters have entered the practical stage, Desruelle said with their specific application examples. "If you want to know the mass distribution underground, you'll use quantum gravity sensors in a variety of activities, including hydrology, seismology, and civil engineering projects that detect crevices, seepage pits, tunnels, and cavities." A lot of people are interested in geodesy instruments," he adds, "so they want to learn more about the Earth's sphere and gravity distribution, and a lot of research institutions are assigned to map gravity nets in different regions." ”
For engineering and geophysical applications, the two different paths of atom fall are only a few millimeters apart, but simply increasing the size of the instrument can greatly improve the sensitivity and can be used to detect unknown invisible matter ( dark matter , dark energy ) that accounts for more than 85 % of the universe's matter.
As of January, the UK's Research and Innovation Foundation has funded seven projects worth £31 million to solve major problems in fundamental physics using quantum technologies. Three of these are the development of quantum-enhanced interferometers and sensors to look for dark matter— probing candidates such as axions or quantization theories to test space-time. Often, disruptive discoveries in science are made on the basis of a fusion of new technologies and established theories, and we may still be at the dawn of quantum sensing, but it has shown us a better path and guided us to explore the deepest mysteries of the universe, which is very fascinating and exciting.
This article is reprinted with permission from the WeChat public account "Chinese Physical Society Journal Network", compiled from Michael Allen. Physics World, 2021, (12): 44, from Physics, No. 1, 2022.
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