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High-stability optical clock based on a continuously ground-state cooled Al+ ion without compromising its accuracy

Fabian Dawel1,2, Lennart Pelzer1, Kai Dietze1,2, Johannes Kramer1,2, Marek Hild1, Steven A. King1,*, Nicolas C. H. Spethmann1, Joshua Klose1, Kilian Stahl1 et al.

Sören Dörscher1, Erik Benkler1, Christian Lisdat1, Sergey G. Porsev3, Marianna S. Safronova3,†, and Piet O. Schmidt1,2,‡

  • *Present address: Oxford Ionics, Oxford OX5 1GN, United Kingdom.
  • †Contact author: msafrono@udel.edu
  • ‡Contact author: Piet.Schmidt@quantummetrology.de
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Phys. Rev. Research 8, 023327 – Published 23 June, 2026

DOI: https://doi.org/10.1103/48zr-2t1p

Abstract

Single-ion optical clocks have shown systematic frequency uncertainties below 10−18, but typically require more than one week of averaging to achieve a corresponding statistical uncertainty. This time can be reduced with longer probe times, but comes at the cost of a higher time-dilation shift due to motional heating of the ions in the trap. We show that sympathetic ground-state cooling using electromagnetically induced transparency of an Al+ clock ion via a cotrapped Ca+ ion during clock interrogation suppresses the heating of the ions. Al+ can be kept close to the motional ground state, independent from the chosen interrogation time, at a relative time-dilation shift of (−1.69±0.20)×10−18. The Ca+ cooling light introduces an additional light shift on the Al+ clock transition of (−9.3±1.1)×10−18. We project that the uncertainty of this light shift can be further reduced by nearly an order of magnitude. This sympathetic cooling enables seconds of interrogation time with 10−19 motional and cooling laser-induced uncertainties for Al+ and can be employed in other ion clocks as well.

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Phys. Rev. Research 8, 023327– Published 23 June, 2026
  • Received 4 November 2025
  • Accepted 27 April 2026
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