Optical clocks with accuracy validated at the 19th digit (2512.07346v1)

Published 8 Dec 2025 in physics.atom-ph

Abstract: We report a comprehensive evaluation of all known sources of systematic uncertainty for two independent $^{{176}$Lu$^+$} single-ion optical references, finding total systematic uncertainty of $1.1\times10^{-19}$ and $1.4\times10^{-19}$ for the two individual systems and $9.6\times10^{-20}$ for the difference. Through direct comparison via correlation spectroscopy, we demonstrate a relative frequency agreement of $-2.4\pm(5.7)\mathrm{stat}\pm(1.0)\mathrm{sys}\times10^{-19}$, where stat' andsys' indicate the statistical and systematic uncertainty, respectively. The comparison uncertainty is statistically limited after approximately 200 hours of averaging with a measurement instability of $4.8\times10^{{-16}(τ/\mathrm{s})^{-1/2}$.}