Absolute frequency measurement of the ${}^2$S${}_{1/2}{\to }^2$F${}_{7/2}$ optical clock transition in ${}^{171}$Yb${}^{+}$ with an uncertainty of $4\times 10^{-16}$ using a frequency link to International Atomic Time

Abstract

The highly forbidden ${}^2$S${}_{1/2}{\to }^2$F${}_{7/2}$ electric octupole transition in ${}^{171}$Yb${}^{+}$ is a potential candidate for a redefinition of the SI second. We present a measurement of the absolute frequency of this optical transition, performed using a frequency link to International Atomic Time to provide traceability to the SI second. The ${}^{171}$Yb${}^{+}$ optical frequency standard was operated for 76% of a 25-day period, with the absolute frequency measured to be 642 121 496 772 645.14(26) Hz. The fractional uncertainty of $4.0\times 10^{-16}$ is comparable to that of the best previously reported measurement, which was made by a direct comparison to local caesium primary frequency standards.

10 pages, 3 figures. Corrected typos - no changes to any numbers or results from version1