A research team from Beihang University, in collaboration with international partners, has announced a major advance in quantum precision measurement, opening new pathways for the detection of ultra-light dark matter (DM) and other new physics. The initiative, named ChangE, is spearheaded by Professor Wei Kai’s group under the direction of Professor Fang Jiancheng at Beihang University.

The collaborative effort includes Peking University, Johannes Gutenberg-Universität Mainz, the Helmholtz Institute Mainz, ETH Zurich, and Nanyang Technological University, among others. Their findings have been published in three high-impact papers: one in Reports on Progress in Physics, and two in Physical Review Letters.

Conventional spin-resonance-based methods for DM detection have been compared to manually tuning a radio across frequencies—a slow and narrow process that is difficult to capture the broad-spectrum signals characteristic of DM. To overcome this, the team developed a novel Hybrid Spin Resonance (HSR) detection technology. This new paradigm enables wide-spectrum sensing of extremely weak signals through precise manipulation of quantum states.

Schematic diagram of DM detection using the Hybrid Spin Resonance (HSR) mode

The team integrated neon, rubidium, and potassium atoms within a finely tuned magnetic field, creating a synergistic effect that significantly enhances both sensitivity and detection bandwidth. This approach improves detection capability by over a thousand fold across a frequency range from 0.01 Hz to 1000 Hz—achieving world-leading sensitivity for axion DM detection within this band.

Moreover, measurements that previously took years can now be completed in just days.

The team also applied this core technology to the search for a hypothetical fifth force, mediated by axion-like particles. To isolate extremely weak signals, researchers developed an atomic co-magnetometer featuring an innovative “Hard Magnetic Core with a Soft Magnetic Shell” structure. Combined with multi-layer magnetic shielding and nuclear spin self-compensation techniques, external interference was reduced by a factor of ten million.

Although no fifth force signal was detected, the experiment established the most stringent constraints on neutron-electron and proton-electron coupling coefficients within the “axion window” (0.01–0.1 meV). At an axion mass of 0.02 meV, sensitivity was improved by more than 10,000 times, providing critical clues in the search for axion DM.

“Both studies center on the precise control of quantum spin systems. Through innovation in co-magnetometry, we have elevated quantum measurement performance to an unprecedented level,” said a team member. The group’s self-developed high-sensitivity measurement apparatus has achieved a quantum energy sensitivity of 10⁻²⁴ eV/Hz¹/²—approximately 1/1024 of the energy of a single visible photon.

In their latest work, the team introduced a new type of atomic spin sensor employing double spin resonance between electron and nuclear spins, enabling traceable and ultra-sensitive measurement of weak magnetic fields. They also established an in situ alkali-noble-gas spin sensor by tracing the measured spin precession frequency to the nuclear spins gyromagnetic ratio constant with high accuracy. The dominant systematic uncertainty induced by Fermi-contact interactions during the trace process has been suppressed by more than 2 orders of magnitude via pulsed train sequences.

This sensor has already been used to search for axion-like particles. Through long-duration signal acquisition, the team achieved a new record for the axion-neutron coupling strength at an axion mass of 6.74×10⁻¹³ eV, outperforming previous limits by about an order of magnitude.

Link to related papers:

https://doi.org/10.1088/1361-6633/adca52

https://doi.org/10.1103/PhysRevLett.134.181801

https://link.aps.org/doi/10.1103/gy6f-4cs9

Editor: Lyu Xingyun