Improved absolute frequency measurement of the strontium ion clock using a GPS link to the SI second

We report on an improved absolute frequency measurement of the 5s2S1/2 – 4d2D5/2 optical transition of a single trapped strontium ion using a Global Positioning System (GPS) link to the SI second. Compared to our previous measurement, the systematic uncertainty of the optical clock has been reduced from 1.5×10−17 to 1.2×10−17 . The measurement campaign was performed over a two-week period in June 2017, with a total measurement time of 92 h. The traceability to the SI second through International Atomic Time was achieved through a GPS link using the Precise Point Positioning method. The dead time uncertainty of the link between the optical clock and the maser was evaluated using standard methods based on a model of the maser noise and on the optical frequency measurement uptimes. The measured frequency of the 88Sr+ ion 5s2S1/2 – 4d2D5/2 transition is 444779044095485.49(19) Hz. This result is in excellent agreement with our previous measurements and the uncertainty has been reduced by almost a factor of four, from a fractional frequency uncertainty of 1.7×10−15 to 4.3×10−16 .

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