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

We report on an improved absolute frequency measurement of the 5s(2 1)S(0)-5s5p(3) P-0 narrowline clock transition at 236.5 nm, for a single, trapped, and laser-cooled In-115 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 x 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 x 10(14). We discuss possibilities for further improvement