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

We report on the first, to the best of our knowledge, frequency ratio measurement of an single-ion clock and a optical lattice clock. A hydrogen maser serves as a flywheel oscillator to measure the ratio by independent optical combs. From 89,000 s of measurement time, the frequency ratio is determined to be 2.952 748 749 874 863 3(23) with relative uncertainty. The measurement creates a new connection in the network of frequency ratios of optical clocks.