Absolute frequency measurement of the In^+ clock transition with a mode-locked laser

Abstract

The absolute frequency of the ${\mathrm{In}}^{+}$ $5s^2 {{}^{1}S}_0–5s5p {{}^{3}P}_0$ clock transition at 237 nm was measured with an accuracy of 1.8 parts in ${10}^{13}$. Using a phase-coherent frequency chain, we compared the ${{}^{1}S}_0–{{}^{3}P}_0$ transition with a methane-stabilized He–Ne laser at $3.39 \mathit{\mu }\mathrm{m}$, which was calibrated against an atomic cesium fountain clock. A frequency gap of 37 THz at the fourth harmonic of the He–Ne standard was bridged by a frequency comb generated by a mode-locked femtosecond laser. The frequency of the ${\mathrm{In}}^{+}$ clock transition was found to be 1 267 402 452 899.92 (0.23) kHz, the accuracy being limited by the uncertainty of the He–Ne laser reference. This result represents an improvement in accuracy of more than 2 orders of magnitude over previous measurements of the line and now stands as what is to our knowledge the most accurate measurement of an optical transition in a single ion.s