Progress toward a single indium ion optical frequency standard

We report an improved absolute frequency measurement of the 5s
<sup>2

1</sup>
S
₀
-5s5p
³
P
₀
narrowline clock transition at 236.5 nm, for a single trapped and laser cooled
¹¹⁵
In
⁺
. Using a narrowline laser as the local oscillator, a linewidth of 43 Hz for the transition is resolved. For absolute frequency measurement we use an optical frequency comb referenced to a cesium clock The transition frequency is found to be 1, 267, 402, 452, 900, 967 (63) Hz, averaged over 13 days of separate measurements. The accuracy is about 5.0 times 10
^-14
. And a preliminary absolute frequency measurement is shown with a Hydrogen maser as the reference. We discuss possibilities for further improvement.