The world's first quantum gravity gradiometer appeared, revealing the underground world for mankind at low cost.

The breakthrough of quantum technology has opened up a new way for human beings to explore the underground world conveniently.

In February 2022, researchers from Birmingham University cooperated with industry partners to demonstrate the world's first quantum gravity gradiometer that can work reliably in the real world, and it can detect underground structures outside strictly controlled laboratory conditions.

technical background

The team of Michael Holinsky, a researcher at Birmingham University in England, developed a gravity gradiometer based on quantum sensors. On February 24th, the related results were published in the journal Nature.

Gravity exists in everything in the world. By measuring the gravity field data on the earth, researchers can determine the structure and density of the lithosphere inside the earth.

The traditional method to obtain the data of the earth's gravity field is to use the classical gravimeter. The classical gravimeter is essentially a highly sensitive spring scale, and the "spring system" will shift with the change of gravity. The researchers measured the change of gravity by observing the displacement.

Needless to say, this measurement method is time-consuming and laborious, and it takes tens of minutes to measure a single data point, so it is difficult to improve the measurement accuracy.

Using quantum properties to measure gravity is an alternative idea. More than 20 years ago, researchers used atomic interferometer to measure gravity field.

However, due to the limitation of environmental factors such as vibration, instrument tilt, interference of magnetic field and thermal field, it is difficult to transform quantum theory into a feasible instrument.

Especially vibration, limits the measuring time of almost all types of gravimeters. If these obstacles can be solved, the measurement will become faster, more comprehensive and cheaper.

For example, the electromechanical gravimeter used in geophysical exploration needs a long measuring time to eliminate local seismic noise or vibration (such as the vibration of passing vehicles or other underground structures), so it is very difficult.

principle of operation

Based on the principle of quantum mechanics, the team of Birmingham University developed a more practical gravity gradiometer.

The working principle of quantum gravity gradiometer is based on the principle of quantum physics, which uses ultra-cold atoms to sense the change of microgravity and measure the subtle change of gravitational field tension when atomic clouds fall. The larger the object, the greater the difference between its density and its surrounding environment, and the greater the difference of measurable tension.

The gravity gradiometer developed by the team of Birmingham University does not directly detect the absolute data of gravity field, but detects the gravity gradient, that is, the rate of change of gravity field intensity in the vertical direction. This kind of measurement can better suppress the noise effect, so it can reflect the local variation of gravity more sensitively.

According to Nature magazine, this kind of quantum gravity gradiometer adopts hourglass structure. Using this structure, they can measure two ultracold rubidium atomic clouds (rubidium is a metal element) vertically separated by one meter, thus obtaining high-precision data.

Schematic diagram of test basic principle

The gravimeter uses a quantum technique called atomic interferometry. Ultra-cold rubidium atoms fall under the influence of gravity and are irradiated by laser beams. The resulting interference patterns depend on their falling speed, and show the intensity and gradient of local gravity field very accurately.

In the field trial operation without preset environment, the quantum gravity gradiometer overcomes the environmental limitations such as vibration. By measuring tiny gravity signals, the quantum gravity gradiometer can sense the situation under the surface without loss. Within 15 minutes, the gravity gradient data of 10 locations can be collected.

On the application level, using this instrument, in the urban environment of Birmingham, England, the equipment detected a 2 -2-meter underground tunnel between two multi-storey buildings. The team also detected a tunnel with a cross-sectional area of 2 square meters under the 8.5-meter street pavement.

Field test concept map

Further scientific improvement of the quantum gravity gradiometer, including the use of large-momentum spectroscope, may further increase the sensitivity of the instrument by 10- 100 times, so that smaller and deeper features can be drawn or detected more quickly. It is predicted that this performance will be realized in the future commercial quantum gravity gradiometer in the next 5- 10 years.

Broad future applications

The quantum gravity gradiometer of Birmingham University is the first instrument in the world that can meet the challenges of real scenes and detect with high spatial resolution, which greatly improves the mapping efficiency of human geological topographic maps.

The greatest technical achievement of this breakthrough is to make the instrument adaptable enough to be used on the road or in the field, not just in the laboratory.

This breakthrough will make various underground and underwater surveys cheaper and more reliable in the future. Therefore, the output speed is increased by 10 times, and the measurement time is reduced from one month to several days. It may open up a series of new application fields for gravity measurement and provide a new perspective for the development of underground space.

Block field test photos

This discovery itself is not a simple academic research progress, but its starting point lies in practical application.

This breakthrough was achieved through the cooperation of RSK Birmingham University (a provider of environmental, engineering and sustainable development solutions), Dstl (a national defense science and technology laboratory under the British Ministry of Defence) and Teledyne e2v. As part of the British National Quantum Technology Program, the project is funded by the UK Research and Innovation Foundation (UKRI) and carried out under the contract of the Ministry of Defence.

Professor Kai Bongs, director of cold atomic physics at the University of Birmingham, UK, and chief researcher at the British Center for Quantum Technology Sensors and Timing, said: "This is an Edison moment in the field of sensing, which will change society, human knowledge and economy. 」

"With this breakthrough, mankind can no longer rely on bad records and luck when exploring, building and repairing buildings and infrastructure. In addition, the academic community is one step closer to the global underground mapping, which will effectively end the situation that human beings know less about the situation a few meters below their feet than Antarctica. 」

Schematic diagram of the depth of underground strata to which the instrument is applicable.

Urban engineers can use quantum gravity gradiometer to detect the near-surface features (underground 10 meters) of some special land, which may affect new buildings, so it can be used to reduce the cost and construction period of railway and highway projects;

Archaeologists may use quantum gravity gradiometer to draw tombs and structures hidden underground, and learn about underground sites without destructive excavation.

Quantum gravity gradiometer can also be used to measure geological features, such as aquifer or soil density, to determine regional water content or discover hidden mineral resources;

Quantum gravity gradiometer can also improve the prediction of natural phenomena such as volcanic eruption.

Block field test photos

Professor George Tuckwell, director of the Department of Earth Science and Engineering at RSK, said, "Detecting the surface conditions such as mine roadways, tunnels and unstable ground is very important for the ability to design, build and maintain housing, industry and infrastructure. The improved ability represented by this new technology can change the way we draw and map global features and complete construction projects. 」

Dr Gareth Brown, technical director of the joint quantum sensing project and senior chief scientist of the British Defense Science and Technology Laboratory (Dstl), said: "For field applications, accurate and rapid measurement of microgravity changes provides new opportunities for detecting previously undetectable objects and navigating more safely in challenging environments. With the maturity of gravity sensing technology, the application of underwater navigation and revealing underground conditions will become possible. 」

The research team of Birmingham University said that once this instrument becomes smaller, more flexible and more sensitive, it will be put into the market as soon as possible.

Although researchers from Birmingham University have discussed the commercialization of the instrument with partner RSK and Teledyne e2v, a subsidiary of American technology group Teledyne in the UK, they are also considering setting up their own startup company to sell the research.

References:

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https://phys . org/news/2022-02-sensor-breakthrough-paves-grounding-world . html