Optical frequency ratio of a ¹⁷¹Yb⁺ single-ion clock and a ⁸⁷Sr lattice clock

We report direct measurements of the frequency ratio 642 THz ${}^2S_{1/2} (F=0)$--${}^2F_{7/2} (F=3)$ electric octupole transition in ${}^{171}\mathrm{Yb}^+$ and 429 ${}^1S_0$--${}^3P_0$ ${}^{87}\mathrm{Sr}$. A series 107 has been performed at Physikalisch-Technische Bundesanstalt between December 2012 October 2019. Long-term variations are larger than expected from individual measurement uncertainties few $10^{-17}$. The cause these remains unknown. Even taking into account, we find a fractional uncertainty $2.5 \times 10^{-17}$, which improves upon previous knowledge by one order magnitude. average is $\nu_{\mathrm{Yb}^+} / \nu_{\mathrm{Sr}} = 1.495\,991\,618\,544\,900\,537(38)$. This represents most accurate two different atomic species to date.