Atomic clock frequency ratios with fractional uncertainty $\leq 3.2 \times 10^{-18}$
Abstract
We report high-precision frequency ratio measurements between optical atomic clocks based on Al, Yb, and Sr. With total fractional uncertainties at or below , these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in Sr ratios at approximately and the Al/Yb ratio at in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key innovation in this work is the use of a common ultrastable reference delivered to all clocks via a 3.6 km phase-stabilized fiber link between two institutions. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2 to 3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of at 1 second. By enabling faster comparisons, this stability will improve sensitivity to non-white noise processes and other underlying limits of state-of-the-art optical frequency standards.
Cite
@article{arxiv.2512.21428,
title = {Atomic clock frequency ratios with fractional uncertainty $\leq 3.2 \times 10^{-18}$},
author = {Alexander Aeppli and Willa J. Arthur-Dworschack and Kyle Beloy and Caitlin M. Berry and Tobias Bothwell and Angela Folz and Tara M. Fortier and Tanner Grogan and Youssef S. Hassan and Zoey Z. Hu and David B. Hume and Benjamin D. Hunt and Kyungtae Kim and Amanda Koepke and Dahyeon Lee and David R. Leibrandt and Ben Lewis and Andrew D. Ludlow and Mason C. Marshall and Nicholas V. Nardelli and Harikesh Ranganath and Daniel A. Rodriguez Castillo and Jeffrey A. Sherman and Jacob L. Siegel and Suzanne Thornton and William Warfield and Jun Ye},
journal= {arXiv preprint arXiv:2512.21428},
year = {2025}
}
Comments
5 pages main text with 4 figures; 1 page and 1 figure end matter; 9 pages and 7 figures supplemental material