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

TL;DR

This paper reports highly precise comparisons between two femtosecond frequency comb systems at NPL, achieving agreement levels as low as 3 x 10^{-21} in optical frequency ratios, advancing the accuracy of optical frequency measurements.

Contribution

It provides the highest reported agreement between Ti:sapphire and Er-doped femtosecond combs, demonstrating their reliability for ultra-precise optical frequency measurements.

Findings

Agreement at 5 x 10^{-18} for optical frequency measurements

Agreement at 3 x 10^{-21} for optical frequency ratios

Traceability to SI second achievable below 1 x 10^{-16}

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$^+$.