Author(s): Alexander Aeppli et al. (BACON Collaboration) 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 $3.2×{10}^{−18}$, these measurements meet an important milestone criterion for redefinit… [Phys. Rev. Lett. 137, 033201] Published Tue Jul 14, 2026