Optical atomic clocks are the most accurate measurement devices ever constructed and have found many applications in fundamental science and technology1,2,3. The use of highly charged ions (HCI) as a new class of references for highest-accuracy clocks and precision tests of fundamental physics4,5,6,7,8,9,10,11 has long been motivated by their extreme atomic properties and reduced sensitivity to perturbations from external electric and magnetic fields compared with singly charged ions or neutral atoms. Here we present the realization of this new class of clocks, based on an optical magnetic-dipole transition in Ar13+. Its comprehensively evaluated systematic frequency uncertainty of 2.2 × 10−17 is comparable with that of many optical clocks in operation. From clock comparisons, we improve by eight and nine orders of magnitude on the uncertainties for the absolute transition frequency12 and isotope shift (40Ar versus 36Ar) (ref. 13), respectively. These measurements allow us to investigate the largely unexplored quantum electrodynamic (QED) nuclear recoil, presented as part of improved calculations of the isotope shift, which reduce the uncertainty of previous theory14 by a factor of three. This work establishes forbidden optical transitions in HCI as references for cutting-edge optical clocks and future high-sensitivity searches for physics beyond the standard model.
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The traces of the frequency ratios are available at https://doi.org/10.5281/zenodo.6901524. Further datasets generated and analysed during this study are available from the corresponding author on request.
All code that has been used to generate or analyse data during this study are available from the corresponding author on request.
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We thank L. Schmöger, M. Schwarz and J. Stark for early contributions to the experimental apparatus, T. Legero for his contributions to the frequency stabilization of the HCI spectroscopy laser, H. Margolis for discussions about the analysis of the frequency data and F. Wolf for comments on the manuscript. A.S. and V.A.Y. thank I. I. Tupitsyn for discussions. The project was supported by the Physikalisch-Technische Bundesanstalt, the Max Planck Society, the Max Planck-Riken-PTB Center for Time, Constants and Fundamental Symmetries, and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through SCHM2678/5-1, SU 658/4-2, the collaborative research centres SFB 1225 ISOQUANT and SFB 1227 DQ-mat, and under Germany’s Excellence Strategy – EXC-2123 QuantumFrontiers – 390837967. These projects 17FUN07 CC4C and 20FUN01 TSCAC have received funding from the EMPIR programme co-financed by the participating states and from the European Union’s Horizon 2020 research and innovation programme. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 101019987). S.A.K. acknowledges financial support from the Alexander von Humboldt Foundation.
The authors declare no competing interests.
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King, S.A., Spieß, L.J., Micke, P. et al. An optical atomic clock based on a highly charged ion. Nature 611, 43–47 (2022). https://doi.org/10.1038/s41586-022-05245-4
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