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.