Frequency ratio of an ${}^{115}$In${}^{+}$ ion clock and a ${}^{87}$Sr optical lattice clock

We report on the first frequency ratio measurement of an ${}^{115}$In+ single ion clock and a ${}^{87}$Sr optical lattice clock. A hydrogen maser serves as a reference oscillator to measure the ratio by independent optical combs. Over more than 90 000 seconds of measurement time, the frequency ratio $f_{\mathrm{I}{\mathrm{n}}^{+}}\mathrm{/}{f}_{\mathrm{S}\mathrm{r}}$ is determined to be 2.952 748 749 874 863 4(21) with relative uncertainty of $7.0\times {10}^{-16}$. The measurement creates a new connection in the network of frequency ratios of optical clocks.