Towards an indium single-ion optical clock

We are investigating the 5s²¹S₀→5s5p ³P₀ transition of a single trapped laser-cooled ¹¹⁵In⁺ ion as a possible optical frequency standard. This line with a natural linewidth of only 1.1 Hz, corresponding to a Q factor of 1.2·10¹⁵, is highly immune to systematic frequency shifts due to external perturbations. For sideband laser cooling and fluorescence detection of the indium ion the 5s²¹S₀→5s5p³P₁ transition at 230.6 nm is excited. Temperatures below 100 μK and a mean vibrational quantum number ⟨n⟩<1 of the ion in the trap have been reached. For the clock transition a resolution of 3·10^-13 (linewidth 350 Hz) has been obtained so far, limited by the laser linewidth. The absolute frequency of the ¹ S₀→³P₀ transition was measured by making a link to known reference frequencies of molecular iodine and methane using a frequency chain