High-resolution spectroscopy of a single ${\mathrm{In}}^{+}$ ion: Progress towards an optical frequency standard

The ${5s}^2{}^1{S}_0\to 5s5p{}^3{P}_0$ transition of a single laser-cooled ${}^{115}{\mathrm{In}}^{+}$ ion in a miniature radiofrequency trap is investigated as a possible optical frequency standard. This line at a frequency of 1267 THz (wavelength 236.5 nm) is highly immune to frequency shifts due to external perturbations so that the expected systematic uncertainties can be reduced to the millihertz level. Experimentally, we obtained a fractional resolution $\Delta \nu /\nu =1.3\mathrm{}\times {10}^{-13}$ (linewidth 170 Hz), limited by the frequency instability of the laser used for excitation. The g factor and the lifetime of the metastable ${}^3{P}_0$ level were measured as ${g(}^3{P}_0)=-9.87(5)\mathrm{}\times {10}^{-4}$ and $\tau {(}^3{P}_0)=0.195\mathrm{}(8)\mathrm{}\hspace{0.5em}\mathrm{s}.$ Both values are in good agreement with perturbative calculations, based on hyperfine mixing between the ${}^{1}P$ and ${}^{3}P$ levels.