Frequency ratio of an 115In+ ion clock and a 87Sr optical lattice clock.

We report on the first, to the best of our knowledge, frequency ratio measurement of an 115In+ singleion clock and a 87Sr 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 fIn/fSr is determined to be 2.952 748 749 874 863 3(23) with 7.7×10-16 relative uncertainty. The measurement creates a new connection in the network of frequency ratios of optical clocks.