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Atomic clocks occupy a unique position in measurement science, exhibitinghigher accuracy than any other measurement standard and underpinning six out ofseven base units in the SI system. By exploiting higher resonance frequencies,optical atomic clocks now achieve greater stability and lower frequencyuncertainty than existing primary standards. Here, we report frequency ratiosof the $^{27}$Al$^+$, $^{171}$Yb and $^{87}$Sr optical clocks in Boulder,Colorado, measured across an optical network spanned by both fiber andfree-space links. These ratios have been evaluated with measurementuncertainties between $6\times10^{-18}$ and $8\times10^{-18}$, making them themost accurate reported measurements of frequency ratios to date. Thisrepresents a critical step towards redefinition of the SI second and futureapplications such as relativistic geodesy and tests of fundamental physics.

Frequency Ratio Measurements with 18-digit Accuracy Using a Network of Optical Clocks