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We report improved frequency ratio measurement with $^{87}$Sr and $^{171}$Yboptical lattice clocks at the National Metrology Institute of Japan (NMIJ). The$^{87}$Sr optical lattice clock is enhanced with several major modificationsand is re-evaluated with a reduced uncertainty of $1.1\times10^{-16}$. Weemployed a $^{171}$Yb optical lattice clock with an uncertainty of$4\times10^{-16}$ that was developed for contributing to International AtomicTime (TAI). The measurement result is $\nu_{\mathrm{Yb}}/\nu_{\mathrm{Sr}}$ =1.207 507 039 343 338 58(49)$_{\mathrm{sys}}$(6)$_{\mathrm{stat}}$ with afractional uncertainty of $4.1\times10^{-16}$, which is 3.4 times smaller thanour previous measurement result.

Improved Frequency Ratio Measurement with 87Sr and 171Yb Optical Lattice Clocks at NMIJ