Hertz-Level Measurement of the Optical Clock Frequency in a Single 88 Sr + Ion

Abstract

The frequency of the 5s ² S _1/2 –4d ² D _5/2 electric quadrupole clock transition in a single, trapped, laser-cooled ⁸⁸ Sr ⁺ 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 ⁸⁸ Sr ⁺ optical clock with stability and reproducibility exceeding that of the primary cesium standard.