Absolute frequency measurement of the 674-nm88Sr+clock transition using a femtosecond optical frequency comb

The frequency of the $5s{}^2{S}_{1/2}$–$4d{}^2{D}_{5/2}$ electric quadrupole transition at 674 nm in a single, trapped, laser-cooled ${}^{88}{\mathrm{Sr}}^{+}$ ion has been measured with respect to the Système International (SI) second using a femtosecond laser optical frequency comb. The measured frequency of 444 779 044 095.52 kHz, with an estimated standard uncertainty of 0.10 kHz, is more accurate than, and in agreement with, the value previously measured using a conventional frequency chain.