High Resolution Spectroscopy of a Single In+ Ion

A single laser cooled ion in a radiofrequency trap can serve as a reference for a highly stable optical frequency standard. We present recent results of our work with single indium ions, using the 5s^{2 1}S₀ - 5s5p ³P₀ transition at a wavelength of 237 nm as the clock transition. This resonance has a linewidth of only 0.8 Hz where systematic shifts should be controllable at the mHz level. A single In⁺ ion is stored in a miniature Paul-Straubel trap and laser cooled to a temperature of about 100 μK. The clock transition is excited using a frequency quadrupled Nd:YAG laser emitting at 946 nm. A fractional resolution of 1.3 × 10^-13 has been achieved so far (linewidth of 170 Hz at 1267 THz), limited by the frequency instability of the Nd:YAG laser. The absolute frequency of the clock transition has been measured with an uncertainty of 2 × 10^-13 using a phase-coherent frequency chain and a methane-stabilized HeNe laser as a reference, that was calibrated against an atomic cesium fountain clock.