Towards an indium single-ion optical clock

We are investigating the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 0/ transition of a single trapped laser-cooled /sup 115/In/sup +/ 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/spl middot/10/sup 15/, is highly immune to systematic frequency shifts due to external perturbations. For sideband laser cooling and fluorescence detection of the indium ion the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 1/ transition at 230.6 nm is excited. Temperatures below 100 /spl mu/K and a mean vibrational quantum number
<1 of the ion in the trap have been reached. For the clock transition a resolution of 3/spl middot/10/sup -13/ (linewidth 350 Hz) has been obtained so far, limited by the laser linewidth. The absolute frequency of the /sup 1/S/sub 0//spl rarr//sup 3/P/sub 0/ transition was measured by making a link to known reference frequencies of molecular iodine and methane using a frequency chain.