Frequency Ratio of Hg199 and Sr87 Optical Lattice Clocks beyond the SI Limit

We report on a frequency ratio measurement of a Hg199-based optical lattice clock referencing a Sr87-based clock. Evaluations of lattice light shift, including atomic-motion-dependent shift, enable us to achieve a total systematic uncertainty of 7.2x10-17 for the Hg clock. The frequency ratio is measured to be vHg/vSr=2.629 314 209 898 909 60(22) with a fractional uncertainty of 8.4x10-17, which is smaller than the uncertainty of the realization of the International System of Units (SI) second, i.e., the SI limit.