High-resolution spectroscopy of a single In ¿ ion: Progress towards an optical frequency standard

ion in a miniature radiofrequency trap isinvestigated as a possible optical frequency standard. This line at a frequency of 1267 THz ~wavelength 236.5nm! is highly immune to frequency shifts due to external perturbations so that the expected systematic uncer-tainties can be reduced to the millihertz level. Experimentally, we obtained a fractional resolution Dn/n51.3310