Absolute frequency measurement of the ${}^{176}$Lu${}^{+}{(}^3{\mathrm{D}}_1)$ standard against the NRC-FCs2 fountain with $2.6\times 10^{-16}$ uncertainty

Abstract

We report an improved absolute frequency measurement of the ${}^{176}$Lu${}^{+}{(}^3{\mathrm{D}}_1)$ optical frequency standard, evaluated via a remote link to the NRC-FCs2 caesium fountain primary frequency standard. Operating a single ion clock with 94.2% uptime over 10 days, and using an ambiguity-resolved precise point positioning (PPP-AR) link over the Global Positioning System (GPS), we determine an absolute frequency of $353638794073800.33(9)$Hz at a fractional uncertainty of $2.6\times 10^{-16}$. This agrees with our previous result, which underpins the CIPM recommended frequency value, and reduces the uncertainty by a factor of 3.6.

8 pages, 4 figures, 4 tables