Recently, scientists from institutions including the University of Science and Technology of China made a fundamental breakthrough in nuclear-spin quantum precision measurement. They developed the first intercity nuclear-spin based quantum sensor network, which experimentally contains the axion topological-defect dark matter and surpasses the astrophysical limits.

Current studies indicate that ordinary visible matter accounts for only about 4.9% of the universe, while dark matter makes up about 26.8%. Axions are among the best-motivated dark matter candidates, and axion fields can form topological defects during phase transitions in the early universe. As the Earth crosses topological defects, the defects are expected to interact with nuclear spins and induce signals. However, detection remains a formidable challenge because signals are extremely weak and short-duration.

To overcome the detection challenge, the research team innovatively developed a nuclear-spin quantum precision measurement that "stores" microsecond-scale axion-induced signals in a long-lived nuclear-spin coherent state, enabling a  minute-scale readout signal. At the same time, the team used nuclear spin as a quantum spin amplification to further enhance the weak dark-matter signal by at least 100-fold, increasing the sensitivity of spin rotation to about 1 μrad, representing an improvement of more than four orders of magnitude over previous techniques.

Furthermore, researchers created the first intercity nuclear-spin based quantum sensor network to discriminate dark matter signals. The network consists of five nuclear-spin quantum sensors geographically distributed across Hefei and Hangzhou with a baseline distance of approximately 320 km, which are synchronized using global positioning system (GPS) time.

Though no statistically significant topological defect crossing event was recorded during two months of observation, the team set the most stringent constraints on the axion-nucleon coupling across an axion mass range from 10 peV to 0.2 μeV, achieving 4.1 × 10¹⁰ GeV at 84 peV.

Notably, as the first laboratory experiment to surpass astrophysical constraints on axion topological-defect dark matter, the study opens up the possibility of examining previously unexplored parameter space. At the interface of quantum precision measurement and fundamental physics, this breakthrough not only provides a new route to probe topological defect dark matter, but also offers a new direction for searches on broad beyond-Standard Model physics such as axion stars and axion strings.

The team is planning to boost sensitivity by another four orders of magnitude in the future by building a global network, extending deployments into space, and developing next-generation technology.