We report high-precision frequency ratio measurements between optical atomic clocks based on ${}^{27}{\mathrm{Al}}^{+}$, ${}^{171}\mathrm{Yb}$, and ${}^{87}\mathrm{Sr}$. With total fractional uncertainties at or below $3.2\times {10}^{-18}$, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in ${}^{87}\mathrm{Sr}$ ratios at approximately $1\times {10}^{-16}$ and the ${\mathrm{Al}}^{+}/\mathrm{Yb}$ ratio at $1.6\times {10}^{-17}$ in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key upgrade from our previous work is the use of a common ultrastable reference delivered to all clocks via a 3.6-km phase-stabilized fiber link between two institutions, enabling better accuracy and stability in frequency transfer. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2–3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of $1.3\times {10}^{-16}$ at 1 s. Identifying individual clock stabilities with a three-corner-hat measurement, we demonstrate the most stable optical lattice and trapped-ion clocks used in a multispecies comparison. By enabling faster comparisons, this stability will improve sensitivity to nonwhite noise processes and other underlying limits of state-of-the-art optical frequency standards.

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• Atomic, optical & lattice clocks
• Time & frequency standards
• Trapped ions
• Optical lattices & traps
• Precision measurements