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

Abstract

We report on an improved absolute frequency measurement of the \SDlong\ 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 $1.5\times10^{-17}$ to $1.2\times10^{-17}$. The measurement campaign was performed over a two-week period in June 2017, with a total measurement time of $92$ hours. The traceability to the SI second through International Atomic Time (TAI) was achieved through a GPS link using the Precise Point Positioning (PPP) 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 \Sr\ ion \SDlong\ transition is $444\,779\,044\,095\,485.49(19)$~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 $1.7 \times 10^{-15}$ to $4.3\times 10^{-16}$.

Noise-induced servo errors in optical clocks utilizing Rabi interrogation

“…The numerical results will be presented for a 88 Sr single-ion clock, studied at the National Research Council (Canada) [18,30], the National Physical Laboratory (UK) [9,31], the Physikalisch-Technische Bundesanstalt (Germany) [11], and by us at VTT [29]. These results are directly applicable also to 40 Ca `and 138 Ba `clocks using Rabi interrogation.…”