Evaluating the performance of the NPL femtosecond frequency combs: Agreement at the $10^{-21}$ level

Abstract

Results are presented from a series of comparisons between two independent femtosecond frequency comb systems at NPL, which were carried out in order to assess their systematic uncertainty. Simultaneous measurements with the two systems demonstrate agreement at the level of 5 x $10^{-18}$ when measuring an optical frequency against a common microwave reference. When simultaneously measuring the ratio of two optical frequencies, agreement at the 3 x $10^{-21}$ level is observed. The results represent the highest reported level of agreement to date between Ti:sapphire and Er-doped femtosecond combs. The limitations of the combs when operating in these two different manners are discussed, including traceability to the SI second, which can be achieved with an uncertainty below 1 x $10^{-16}$. The technical details presented underpin recent absolute frequency measurements of the ${}^{88}$Sr${}^{+}$ and ${}^{171}$Yb${}^{+}$ optical clock transitions at NPL, as well as a frequency ratio measurement between the two optical clock transitions in ${}^{171}$Yb${}^{+}$.

11 pages, 6 figures

References in corpus (2)

• Frequency ratio of two optical clock transitions in ${}^{171}$Yb${}^{+}$ and constraints on the time-variation of fundamental constants
• Testing ultrafast mode-locking at microhertz relative optical linewidth

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• 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
• Thermal-light heterodyne spectroscopy with frequency comb calibration
• International Optical Clock Comparison Using the European Optical Fiber Network