Researchers led by Professor Lu Zhengtian and Researcher Xia Tian at the University of Science and Technology of China (USTC) and Hefei National Laboratory have built a cold-atom comagnetometer that traps two ytterbium isotopes in the same 'optical cage,' using them as mutual references. Common magnetic-field fluctuations are suppressed by more than 30,000x, and spin coherence times extend to 60 seconds. The work was published in *Nature Photonics* on August 23 — turning 'find signal vs. subtract background' into 'two independent atomic species perturbed by the same field, checked against each other for deviations.'
Two Atoms in One Optical Lattice
The core challenge of precision magnetometry: both the signal of interest and ambient magnetic fields shift atomic spin precession frequencies. Observing a single species, one cannot tell them apart.
The team confined Yb-171 (spin-1/2) and Yb-173 (high-spin) simultaneously in the same one-dimensional optical lattice. Both species sit at the same location and experience nearly identical background fields — when the field changes, both precession frequencies shift, but in different proportions determined by each isotope's nuclear magnetic properties, a natural 'fingerprint.'
If both frequencies change in the expected ratio, it is most likely common magnetic noise; a deviation from that ratio indicates a signal worth pursuing. This is the principle of a comagnetometer.
Cat States as Engineering, Not Gimmicks
Comagnetometers are not new; they typically use centimeter-scale glass vapor cells at room temperature. But cells with thick walls prevent atoms from approaching the micron distances relevant to hypothesized short-range interactions (fifth forces, axion dark matter, electron EDM).
The new experiment uses cold atoms in a laser-formed 1D optical lattice to solve the distance problem. However, lattice-laser-induced tensor light shifts rapidly dephase high-spin systems, destroying spin coherence.
The solution: put Yb-173 into a Schrödinger spin cat state — a quantum superposition whose precession frequency is insensitive to tensor light shifts. This symmetry cancels systematic decoherence. Yb-171, with its spin-1/2 nucleus, is naturally immune to tensor light shifts.
The key novelty: for the first time in a cold-atom system, the team combined the dual-isotope approach with cat states, achieving a magnetic noise suppression factor above 30,000.
What the Three Numbers Mean
| Metric | Value | Physical meaning | |---|---|---| | Magnetic noise suppression | >30,000 | Common-mode background field 'blindness' reduced to 1/30,000 of prior levels | | Spin coherence time | 60 s | Maximum phase retention — determines the weakest detectable signal (~0.017 Hz resolution) | | Spatial resolution | Micron scale | Set by lattice spacing — 4 orders of magnitude smaller than vapor cells |
The 60-second coherence time reaches the frequency band characteristic of axion dark matter and ultralight dark photons. The micron-scale resolution enables probing short-range forces that previously had no suitable tool. The team also precisely measured the nuclear magnetic moment ratio of Yb-171 to Yb-173 — a fundamental constant previously only inferred indirectly — now measured directly under identical trapping conditions.
Applications: Dark Matter, Fifth Forces, Electron EDM
1. Axion dark matter: ultralight candidates coupling to magnetic fields produce monochromatic electromagnetic waves within the device's frequency sensitivity range. 2. Fifth forces: many theories predict interactions acting only at sub-millimeter to micron scales — inaccessible to vapor cells, but well matched to optical lattices. 3. Electron electric dipole moment (EDM): linked to the matter-antimatter asymmetry of the universe; comagnetometry is among the most competitive precision frontiers.
No new-physics signal was directly observed, but the work establishes an experimental platform that previously could not exist — enabling new tests that can be verified or falsified on a tabletop.
Context Within China's Quantum Precision Measurement Landscape
- In January, USTC's Peng Xinhua and Jiang Min teams published in *Nature* the first nucleus-spin-based quantum sensing network.
- On August 23, the Lu/Xia team achieved 30,000x suppression + 60 s coherence in a comagnetometer.
- On August 24, China's proposal for an international standard on quantum entropy-source randomness quality and testing was approved at the fifth IEC/ISO JTC3 plenary session; China is also leading five related international standards, including performance evaluation of free-fall cold-atom absolute quantum gravimeters.
- Three ISO/IEC quantum technology international standards have been published to date, all led by China.
- Fundamental physics: the dual-isotope + cat-state framework could extend to three isotopes, with 1–2 orders of magnitude in precision headroom.
- Industrialization: the newly approved IEC/ISO standard initiatives open a standards-setting window and clarify compliance roadmaps.
- Dark matter searches: the device enables tabletop axion searches, complementing underground detectors like PandaX-4T.
- Quantum computing/communication: entanglement between different nuclear spins in one lattice could support distributed quantum computing.
- China Science Daily via Sina Finance, 'Development of a Cold-Atom Comagnetometer' (Aug 23)
- Tencent News, 'Interference Reduced 30,000-fold: Two Atom Ensembles Compare Clocks, Inviting Schrödinger's Cat' (Aug 24)
- Nature Photonics original paper (online Aug 23)
- 10jqka, 'New Progress in International Standards for Quantum Technology' (Aug 24)
- USTC official team introductions (Lu Zhengtian, Xia Tian, Zhang Jiehang)
Together, these advances mark China's shift from isolated breakthroughs toward systematic international standards-setting in quantum precision measurement.