AN OPTICAL CLOCK BASED ON A SINGLE TRAPPED ⁸⁸Sr⁺ ION

The 5s ²S_1/2-4d ²D_5/2 electric quadrupole transition in a single trapped and laser-cooled ⁸⁸Sr⁺ ion provides the reference for a highly stable and accurate optical clock. This paper describes recent progress on the ⁸⁸Sr⁺ optical clocks under development at NPL. An ultrastable laser at 674 nm is used to excite the clock transition and a fractional frequency resolution of 2 × 10^-14 has been achieved, corresponding to a linewidth of 9 Hz. A relative frequency instability of 2 × 10^-15 at 5000 s has been observed in the comparison between two similar systems and the fractional uncertainty in the absolute frequency of the clock transition is 3.8 × 10^-15. Improvements to the experimental arrangement are in progress and are expected to lead to a ⁸⁸Sr⁺ optical clock with stability and reproducibility exceeding that of the primary caesium standard.