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Precision gravity measurement technology has great significance in many fields such as resource exploration, autonomous navigation, geological structure and seismic research (Figure 1), and quantum gravity measurement technology based on atomic interference has the characteristics of high precision, low drift, no mechanical wear, etc., and has gradually matured and shown superiority over classical instruments in many aspects. The atomic interferometer team of the Institute of Precision Measurement of the Chinese Academy of Sciences briefly reviewed and introduced the development process and the latest progress of atomic interferometry gravity measurement technology, and at the same time, sorted out and analyzed the problems and possible solutions that are still facing.
Figure 1 High-precision atomic gravity gradient meter can be used to detect underground features such as caves, tunnels, and mineral deposits, and the typical signal size of gravity gradient anomalies generated by different features is shown
Gravity measuring instruments include a gravimeter that directly measures the acceleration of gravity and a gravity gradient that measures the second-order differential of the gravitational position, which are sensitive to the long-wave gravity field (the space-slow-change gravity field generated by distant objects) and the short-wave gravity field (the space-fast-changing gravity field generated by a close-range object). Functionally, the two can be divided into an absolute gravity meter that measures the absolute gravity value and a relative gravity meter that measures the amount of gravity change, as well as an absolute gravity gradient meter that measures the absolute gravity gradient value and a relative gravity gradient meter that measures its change. Absolute instruments have higher long-term stability, can calibrate the long-term drift of relative instruments, and relative instruments generally have better reliability, can meet the needs of aviation, navigation and other harsh conditions. Absolute gravity measuring instruments that can meet the measurement needs of harsh conditions have long been expected, and the advent of atomic interferometric gravity measurement technology has made this expectation possible.
Atomic interferometers use lasers to manipulate the material waves of atoms to divide, reflect, and combine beams in space, and in the process write the phase of the laser into the phase of the atom's material waves. Since atoms have rest mass and their trajectories are deflected in the gravitational field, picking up different laser phases, we can achieve precise measurements of gravity by measuring the phase of the atomic interference signal (Figure 2). Because the atoms as the measuring medium have an extremely stable energy level structure, the atomic interferometer has the same precision and stability as the atomic clock, and at the same time has the advantages of no mechanical wear and long-term continuous operation.
Fig. 2 Atomic gravity meter (left) and atomic gravity gradient meter (right) implemented based on atomic interferometer, which consists of two or more atomic gravimeters spaced at a specific distance apart
After 30 years of development, atomic gravity meters have a high degree of technical maturity. In the 2017 international comparison of absolute gravimeters, 6 atomic absolute gravimeters from China participated in the comparison, of which 4 gravimeters reached the same level as the most advanced classical gravimeter in the world and were adopted by the organizing committee. NASA's GILAFE atomic gravimeter achieves maritime and aeronautical gravity measurements respectively, and the measurement accuracy exceeds the classical relative gravimeter measured by peers. At present, the absolute gravimeter products on the market are mainly the falling absolute gravimeter of the American Microg company, the atomic gravimeter as a new generation of quantum gravity sensor has also been commercialized, Muquans and Zhongke Kuyuan and other domestic and foreign companies have launched their own atomic absolute gravimeter products.
Fig. 3 FG5-X type classical absolute gravimeter and WAG-C5 type atomic absolute gravimeter
At the beginning of the design of the atomic gravity gradient meter, the goal was to achieve high-precision measurement of the gravitational constant G as a scientific research instrument, so it was slightly later in the practical direction. But several pieces of work published in early 2022 marked an equally high level of reliability, resolution, accuracy, and integration of the atomic gravity gradient. The University of Birmingham in the United Kingdom realized a highly reliable atomic gravity gradient meter based on the optical design of the hourglass, and observed a gravity gradient signal of 170 E (1E ≈ 10-10 g/m) above a tunnel with a signal-to-noise ratio of 3 times. The French company iXblue has developed the world's first sub-E level transportable atomic gravity gradient meter, with a measurement resolution of 0.15 E. The Institute of Precision Surveying of the Chinese Academy of Sciences reported a highly integrated sub-E-level atomic absolute gravity gradient meter with an unprecedented probe volume of 92 L and completed the most detailed systematic error assessment work to date. However, in terms of dynamic measurement, atomic gravity gradient meters still face problems such as insufficient scale factors and low phase extraction efficiency, which need to be solved by the application of innovative technologies such as large momentum transfer and shear interference imaging.
Summary and outlook
Atomic interferometer and gravity gradient instrument with high precision, low drift, no mechanical wear and other advantages, is a new generation of precision gravity measuring instruments with great application potential. At present, the atomic gravimeter has a high degree of maturity and has fully demonstrated the performance of classical instruments in static and dynamic scenarios. Atomic gravity gradient meters have also made a series of important progress and reached a high level in terms of reliability, resolution, accuracy and integration in recent years, but there are still some problems in the dynamic measurement scenario, which are expected to be solved based on existing innovative technologies.