Absolute frequency measurement of the ⁸⁸ Sr ⁺ clock transition using a GPS link to the SI second

We report the results of a recent measurement of the absolute frequency of the 5 s 2 S 1 / 2 – 4 d 2 D 5 / 2 transition of the 88 S r + ion. The optical frequency was measured against the international atomic time realization of the SI second on the geoid as obtained by frequency transfer using a global positioning system link and the precise point positioning technique. The measurement campaign yielded more than 100 h of frequency data. It was performed with improvements to the stability and accuracy of the single-ion clock compared to the last measurement made in 2012. The single ion clock uncertainty is evaluated at 1.5 × 10 − 17 when contributions from acousto-optic modulator frequency chirps and servo errors are taken into account. The stability of the ion clock is 3 × 10 − 15 at 1 s averaging, a factor of three better than in the previous measurement. The results from the two measurement campaigns are in good agreement. The uncertainty of the measurement, primarily from the link to the SI second, is 0.75 Hz ( 1.7 × 10 − 15 ). The frequency measured for the S – D clock transition of 88