Improved absolute frequency measurement of the 115In+ 5s 2 1 S 0-5s5p 3 P 0 narrowline transition: Progress towards an optical frequency standard

We report on an improved absolute frequency measurement of the 5s 2 1 S 00-5s5p 3 P 0 narrowline clock transition at 236.5 nm, for a single, trapped, and laser-cooled 115In ion. Using a narrowline laser as the local oscillator, a linewidth of 43 Hz for the transition is resolved. The uncertainty of the transition frequency’s centroid is 18 Hz, leading to a fractional uncertainty of 1.4 × 10−14. For absolute frequency measurement, we use an optical frequency comb locked to a cesium clock as the reference. The transition frequency is found to be 1267402452900967(63) Hz, averaged over 13 days of separate measurements. The accuracy is about 5.0 × 10−14. We discuss possibilities for further improvement.