Abstract
Quantum electrodynamics has been tested to accuracies below the parts-per-trillion level in light-mass systems. However, tests in heavy-mass systems with a large nuclear charge have not yet reached similar accuracy. Here we report the hyperfine-structure splitting in the 1s ground state of radioactive hydrogen-like 208Bi82+. We produced the isotope in a nuclear reaction and injected the beam into a storage ring to perform laser spectroscopy on samples of 105 ions of Bi82+ that have only a single remaining electron, which experiences extreme magnetic-field strengths. Our result for the hyperfine splitting is in excellent agreement with the most accurate prediction based on a combination of quantum electrodynamics calculations with an empirical treatment of the hyperfine-structure anomaly ratio extracted from laser spectroscopy on neutral atoms of 209Bi and 208Bi. This achievement paves the way for the most stringent test of quantum electrodynamics in strong magnetic fields and demonstrates the feasibility of laser spectroscopy on other exotic ions with low production yields.
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Data availability
The datasets generated during the beamtime and analysed for the current study are subject to further analysis. They are available from the corresponding authors on reasonable request and will be made publicly available and linked to 10.26083/tuprints-00026367 at a later stage.
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Acknowledgements
The results presented here are based on experiment E128, which was performed at the ESR at the GSI Helmholtzzentrum fuer Schwerionenforschung, Darmstadt (Germany), in the frame of FAIR Phase-0 and the SPARC Collaboration. W.N. thanks L. Skripnikov for valuable discussions. We acknowledge financial support from the German Ministry for Education and Research under grants 05P21RDFA1 (M.H., W.N. and S.R), 05P21PMFA1 (V.H., K.U. and C.W.) and 05P21RDFN1 (K.M. and W.N.), from the State of Hesse within the Research Cluster ELEMENTS Project ID 500/10.006 (C.B., J.G., Y.A.L. and T.S.) and from the Canadian Natural Sciences and Engineering Research Council (NSERC) via grant SAPIN-2019-00030 (G.L.). M.H., P.I., P.M. and S.R. acknowledge support from HGS-HiRe.
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Contributions
The experiment was planned by W.N., R.S., C.B. and T.S. The laser system was set up and maintained by R.S. Data acquisition was prepared by C.B., K.M. and U.S. High-voltage measurements were performed by J.M., S.P., J.R. and K.K. The detection systems were prepared by C.B., V.H., W.N., T.R. and C.W. Storage-ring optimization and operation was performed by R.H., S.L., B.L., J.R. and M.S. The Schottky measurement system was set up and operated by C.B., R.J.C. and S.S. The experiment was prepared and performed by Z.A., C.B., R.J.C., C.G., G.L., D.F.F., J.G., V.H., M.H., P.I., S.K., K.K., Y.A.L., W.N., K.M., P.M., T.R., S.R., R.S., R.S.S., S.S., K.U. and D.W. The data analysis was performed by M.H. and discussed among all authors. The paper was written by M.H. and W.N. All authors read and approved the paper.
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Extended data
Extended Data Fig. 1 Accumulated PMT signal for all bunched-beam measurements with 209Bi82+ and 208Bi82+ in relation to the time for one revolution in the ESR.
Accumulated PMT signal for all bunched-beam measurements with 209Bi82+ and 208Bi82+ in relation to the time for one revolution in the ESR (≈ 500 ns). The data labeled “signal” was recorded immediately after a laser pulse for a sampling time of 2.35 ms (209Bi) or 1.54 ms (208Bi). The background data was collected 20 ms after the pulse for the same sampling time, when essentially all bismuth ions had returned to the ground state. The falling slope of detected events was independent of the laser pulse and persisted throughout the revolution period. It must therefore originate from a local source in the detection region and is attributed to the deexcitation of residual gas molecules after collisional excitation by the fast ions.
Source data
Source Data Fig. 3
Raw data of the resonance spectra. Uncertainty in sqrt(counts).
Source Data Fig. 5
Individual measurements of the hyperfine transition wavelength in 208Bi82+ with uncertainties.
Source Data Extended Data Fig. 1 and Table 1
Accumulated PMT counts for all bunched-beam measurements with 209Bi82+ and 208Bi82+.
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Horst, M., Andelkovic, Z., Brandau, C. et al. Storage-ring laser spectroscopy of accelerator-produced hydrogen-like 208Bi82+. Nat. Phys. 21, 1057–1063 (2025). https://doi.org/10.1038/s41567-025-02885-x
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DOI: https://doi.org/10.1038/s41567-025-02885-x
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