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arXiv:2606.23151

An Al$^+$ clock with $1.6\times10^{-18}$ systematic uncertainty and its frequency ratios

Fabian Dawel, Johannes Krämer, D. Drapier, Lennart Pelzer, Kai Dietze, M Haider Ali, Marek B. Hild, Vincent Barbé, S A King, J. Klose, Kilian Stahl, Johannes Rahm, Navraj Poudel, Sören Dörscher, S. Weyers, Erik Benkler, Christian Lisdat, Piet O. Schmidt

Published Jun 22, 2026• 0 citations
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Abstract

Advances in optical clocks motivate a redefinition of the second, requiring rigorous evaluations of systematic uncertainties and robust consistency among the clocks. Here, we report the full evaluation of the systematic frequency shifts of an $^{27}\mathrm{Al}^+$ single-ion clock, and the measurement of its absolute frequency and frequency ratio with a $^{87}$Sr optical lattice clock at PTB. The evaluated total systematic fractional frequency uncertainty is $1.6\times10^{-18}$, mainly limited by the accuracy of the relevant atomic coefficients and by background gas collisions. The absolute frequency of the clock has been measured to be $ν_{\mathrm{Al}^+}=1 121 015 393 207 859.19(24)\,$Hz, obtained by comparison with two primary caesium fountain clocks at PTB. The frequency ratio between the Al and Sr optical clocks has been determined to be $ν_{\mathrm{Al}^+}/ν_{^{87}\mathrm{Sr}}=2.611 701 431 781 462 668(36)$, limited by the accuracy of the Sr clock. This ratio differs by $8.6σ$ and $1.2σ$ from the 2021 and 2025 frequency ratio published by the BACON collaboration, respectively. These results represent an important contribution toward a future redefinition of the second using optical clocks, and underscore the importance of independent measurements of clock-candidate frequency ratios across different institutions.

DOI
10.48550/arxiv.2606.23151

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Advances in optical clocks motivate a redefinition of the second, requiring rigorous evaluations of systematic uncertainties and robust consistency among the clocks. Here, we report the full evaluation of the systematic frequency shifts of an 27Al+^{27}\mathrm{Al}^+27Al+ single-ion clock, and the measurement of its absolute frequency and frequency ratio with a 87^{87}87Sr optical lattice clock at PTB. The evaluated total systematic fractional frequency uncertainty is 1.6×10−181.6\times10^{-18}1.6×10−18, mainly limited by the accuracy of the relevant atomic coefficients and by background gas collisions. The absolute frequency of the clock has been measured to be νAl+=1121015393207859.19(24) ν_{\mathrm{Al}^+}=1 121 015 393 207 859.19(24)\,νAl+​=1121015393207859.19(24)Hz, obtained by comparison with two primary caesium fountain clocks at PTB. The frequency ratio between the Al and Sr optical clocks has been determined to be νAl+/ν87Sr=2.611701431781462668(36)ν_{\mathrm{Al}^+}/ν_{^{87}\mathrm{Sr}}=2.611 701 431 781 462 668(36)νAl+​/ν87Sr​=2.611701431781462668(36), limited by the accuracy of the Sr clock. This ratio differs by 8.6σ8.6σ8.6σ and 1.2σ1.2σ1.2σ from the 2021 and 2025 frequency ratio published by the BACON collaboration, respectively. These results represent an important contribution toward a future redefinition of the second using optical clocks, and underscore the importance of independent measurements of clock-candidate frequency ratios across different institutions.
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