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 S1/22(F=0)-F7/22(F=3) electric octupole transition in 171Yb+ and the 429 THz S01-P03 transition in 87Sr. 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 × 10−17, which improves upon previous knowledge by one order of magnitude. The average frequency ratio is νYb+/νSr=1.495991618544900537(38). This represents one of the most accurate measurements between two different atomic species to date.