A single indium ion optical frequency standard

A single indium ion stored in a radio-frequency trap and laser-cooled to a temperature below 1 mK is a promising candidate for an optical frequency standard of exceptionally high accuracy and stability. Detection and laser cooling of In⁺ are performed using the 5s^{2 1}S₀ - 5s5p³P₁ intercombination line at 231 nm. The dye laser formerly used was recently replaced by a diode-based laser system allowing for long-term operation of the standard. Loading of the trap is now realized by resonant 2-photon ionization of indium at 410 nm. This strongly reduces deposition of indium on the trap electrodes so that contact potentials close to the trapped ion can be avoided.The frequency was determined to 1 267 402 452 899.92 (0.23) kHz (Deltav / v = 1.8 x 10^-13), where the measurement uncertainty is dominated by the frequency uncertainty of the He-Ne laser. Further improvements in this measurement together with a comparison with other narrow atomic resonances would allow investigations of variations of fundamental constants in time.