Indium single-ion optical frequency standard

We are investigating the 5s21S0→5s5p 3P0 transition of a single trapped laser-cooled 115In+ ion as a candidate for an optical frequency standard. This line with a natural linewidth of only 0.8 Hz is highly immune to systematic frequency shifts. For sideband laser cooling and fluorescence detection of the indium ion the 5s21S0→5s5p 3P1 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 1.3.10-13 (linewidth 170 Hz) has been obtained so far, limited by the short term frequency fluctuations of the clock laser. The absolute frequency of the 1S0→3P0 transition was measured by making a link to the reference frequency of the methane-stabilised HeNe laser using a frequency chain