Absolute frequency measurement of 115In+ clock transition

We report recent improvement in absolute frequency measurement of the
¹¹⁵
In
⁺
5s
<sup>2

1</sup>
S
₀
-5s5p
³
P
₀
clock transition at 236.5 nm. Using a narrowline laser as a 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
. In the experiment, a single
¹¹⁵
In
⁺
is trapped in a three-ring Paul-Straubel trap and laser cooled to the trap's vibrational ground states.