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

Abstract

The absolute frequency of the In${}^{+}$ $5s^{21}{S}_0$ - $5s5p^3{P}_0$ 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 $\mu$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 In${}^{+}$ clock transition was found to be $1267402452899.92(0.23)$ kHz, the accuracy being limited by the uncertainty of the He-Ne laser reference. This represents an improvement in accuracy of more than 2 orders of magnitude on previous measurements of the line and now stands as the most accurate measurement of an optical transition in a single ion.

3 pages, 2 figures. accepted for publication in Opt. Lett