Frequency Ratio of ¹⁹⁹Hg and ⁸⁷Sr Optical Lattice Clocks beyond the SI Limit

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We report on a frequency ratio measurement of a ¹⁹⁹Hg-based optical lattice clock referencing a ⁸⁷Sr-based clock. Evaluations of lattice light shift, including atomic-motion-dependent shift, enable us to achieve a total systematic uncertainty of 7.2 × 10⁻¹⁷ for the Hg clock. The frequency ratio is measured to be ν Hg/ν Sr=2.629 314 209 898 909 60(22) with a fractional uncertainty of 8.4 × 10⁻¹⁷, which is smaller than the uncertainty of the realization of the SI second, i.e., the SI limit.