Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion

The frequency of the 5S S-2(1/2)-4d D-2(5/2) electric quadrupole clock transition in a single, trapped, laser-cooled Sr-88(+) ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements' required to obtain a cesium-limited frequency measurement are described and are expected to lead to a Sr-88(+) optical clock with stability and reproducibility exceeding that of the primary cesium standard.