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

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

Automated summary

The study reports direct measurements of the frequency ratio between different atomic species with unprecedented accuracy, but long-term variations remain unexplained. The fractional uncertainty in the frequency ratio has significantly improved, providing an important foundation for future experiments and research.