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

Single laser-cooled ions stored in radiofrequency traps are the atomic syst ems which allow the highest resolution in optical or microwave spectroscopy . A narrow transition in such an ion can serve as a reference for a frequen cy standard of extremely high accuracy and stability. In view of this appli cation we study the 5s(2) S-1(0)-5s5p P-3(0) clock transition in a single t rapped In-115(+) ion at a wavelength of 237 nm [1]. This transition is high ly immune to systematic frequency shifts. A frequency control at the millih ertz level is expected leading to a residual relative uncertainty at the le vel of 10(-18). For realizing the standard it is necessary to compare its a bsolute frequency to other known frequencies, at best to the present primar y frequency standard, the cesium atomic clock. We report on the comparison of the In+ clock transition to a methane-stabilized He-Ne laser at 3.39 mum . This laser was calibrated before the measurement 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 fem tosecond laser. The frequency of the clock transition was determined to 126 7402452899.92 (0.23) kHz where the accuracy of the measurement is limited b y the uncertainty of the He-Ne standard.