Abstract
The lowest detectable abundances with accelerator mass spectrometry remain limited by persistent background interferences from atomic and molecular isobars as well as neighbouring isotopes. Such interference is eliminated by laser-based atom trap trace analysis that captures individual atoms through resonant photon scattering. Its detection limit solely depends on the atom counting rate and data acquisition duration. Here we report the direct detection of atmospheric 42Ar at an isotopic abundance level of 10−21 by combining atom trap trace analysis with an isotope pre-enrichment process, achieving a detection limit several orders of magnitude beyond existing methods. Our measurement consumed only 10 l of argon at standard temperature and pressure. This result demonstrates a powerful tool for detecting isotopes at previously inaccessible abundance levels, with implications for environmental dating and background characterization in next-generation liquid-argon detectors.
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Acknowledgements
We thank W.-H. Wang, M.-J. Zhang, Y.-J. Zhang and J.-Y. Wu for their assistance in the extraction of pre-enriched samples. We thank H. Li for his assistance in the mass spectrometry measurements. This work is funded by the Innovation Program for Quantum Science and Technology (2021ZD0303101), National Natural Science Foundation of China (12474269, 41727901, 12205343 and 12025506) and Chinese Academy of Sciences.
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Z.-F.W., J.W.L., W.J., Z.-T.L. and G.M.Y. developed the ATTA system. Z.H.J. and L.T.S. developed the pre-enrichment system. J.W.L. and Z.H.J. completed the enrichment of 42Ar. Z.-F.W. performed the ATTA measurements of 42Ar abundance. All authors contributed to the discussion of the results and writing of the paper.
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Nature Physics thanks Niels Bidaultand, Béla Majorovits and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data
Extended Data Fig. 1 Schematic diagram of ATTA apparatus for argon isotopes.
The argon beam is excited to the metastable 4 s[3/2]2 state in the RF discharge source. Metastable argon atoms are transversely cooled and focused through pre‑collimation, transverse cooling (TC), and a 2D‑MOT, then longitudinally decelerated in a Zeeman slower (ZSL), and finally captured in a magneto‑optical trap (MOT). Arrows in the diagram indicate the 811.8 nm laser beams.
Extended Data Fig. 2 Energy level diagram of argon relevant to the ATTA measurement.
The red arrow denotes the 811.8 nm transition used for laser cooling, trapping, and detection of metastable argon atoms. The orange arrow indicates the quench transition, which de‑excites metastable atoms to the ground state.
Extended Data Fig. 3 Dependence of the 38Ar loading rate on the MOT laser detuning.
The loading rate of 38Ar is normalized to the maximum value achieved at the optimal detuning of -14.5 MHz. The error bars denote one standard deviation.
Supplementary information
Supplementary Information (download PDF )
Supplementary Sections 1–10, Figs. 1–9, Tables 1 and 2, and discussion.
Source data
Source Data Fig. 1 (download XLSX )
Source data for the simulated and measured Ar+ beam profiles on the target plane.
Source Data Fig. 2 (download XLSX )
Source data for the single-atom fluorescence signal of 42Ar.
Source Data Fig. 3 (download XLSX )
Source data for the single-atom signal versus MOT detuning and MOT laser power.
Source Data Fig. 4 (download XLSX )
Source data for the the isotopic abundance of 42Ar in the atmosphere.
Source Data Extended Data Fig./Table 3 (download XLSX )
Source data for the the 38Ar loading rate versus the MOT laser detuning.
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Wan, ZF., Liang, J.W., Jia, Z.H. et al. Detection of atmospheric 42Ar at the 10−21 level by atom counting. Nat. Phys. (2026). https://doi.org/10.1038/s41567-026-03257-9
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DOI: https://doi.org/10.1038/s41567-026-03257-9
