Absolute frequency measurement of the 4355-nm ^171Yb^+-clock transition with a Kerr-lens mode-locked femtosecond laser

We have measured the frequency of the $6s {}^2{S}_{1/2}-5d {}^2{D}_{3/2}$ electric-quadrupole transition of ${}^{171}{\mathrm{Yb}}^{+}$ with a relative uncertainty of $1\times {10}^{-14}$, ${\nu }_{\mathrm{Yb}}=688 358 979 309 312 \mathrm{Hz}\pm 6 \mathrm{Hz}$. We used a femtosecond frequency comb generator to phase-coherently link the optical frequency derived from a single trapped ion to a cesium-fountain-controlled hydrogen maser. This measurement is one of the most accurate measurements of optical frequencies ever reported, and it represents a contribution to the development of optical clocks based on a ${}^{171}{\mathrm{Yb}}^{+}$-ion standard.