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

The absolute frequency of the In+ 5s(2) S-1(0)-5s5p P-3(0) clock transition at 237 nm was measured with an accuracy of 1.8 parts in 10(13). Using a ph ase-coherent frequency chain, we compared the S-1(0)-P-3(0) transition with a methane-stabilized He-Ne laser at 3.39 mum, which was calibrated against an atomic cesium fountain clock. A frequency gap of 37 THz at the fourth h armonic 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 1267 402 452 899.92(0.23) kHz, the accuracy being limited b y the uncertainty of the He-Ne laser reference. This result represents an i mprovement in accuracy of more than 2 orders of magnitude over previous mea surements of the line and now stands as what is to our knowledge the most a ccurate measurement of an optical transition in a single ion.s (C) 2000 Opt ical Society of America.