Atomic clock frequency ratios with fractional uncertainty ≤ 3.2 × 10^-18

2025/12/24 by Alexander Aeppli, Aeppli, Alexander, Willa J. Arthur-Dworschack +55
Physics and Astronomy · Engineering · #Advanced Frequency and Time Standards #Atomic and Subatomic Physics Research #GNSS positioning and interference

paper · pdf · doi:10.48550/arxiv.2512.21428

Abstract

We report high-precision frequency ratio measurements between optical atomic clocks based on ²⁷Al⁺, ¹⁷¹Yb, and ⁸⁷Sr. With total fractional uncertainties at or below 3.2 × 10^-18, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in ⁸⁷Sr ratios at approximately 1×10^-16 and the Al⁺/Yb ratio at 1.6×10^-17 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 1.3 × 10^-16 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.

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