Absolute frequency measurement and high resolution spectroscopy of 115In+ 5s21S0–5s5p 3P0 narrowline transition

We measure the frequency of the 5s21S0–5s5p 3P0 narrowline clock transition at 236.5 nm, for a single, trapped and laser cooled 115In+ ion. In the experiment, an ultra-narrow linewidth laser (<1.34 Hz at 3 s integration time) is used to interrogate the clock transition for high resolution spectroscopy. A linewidth of 43 Hz of the clock transition is observed. The uncertainty of the line centroid is 18 Hz, leading to a fractional uncertainty of 1.4×10-141.4×10-14. The frequency is measured by using an optical frequency comb referenced to a cesium clock. The transition frequency is found to be 1, 267, 402, 452, 901.265 (256) kHz, averaged over 13 days of separate measurement. The accuracy of 2.35×10-132.35×10-13 is due to the reference cesium clock calibrated against UTC time. We discuss ways for further improvements.