Towards an indium single-ion optical frequency standard

A narrow transition of a single laser-cooled ion, stored in a radio-frequency trap, can serve as a reference for a frequency standard of very high accuracy and stability. For the implementation of such a system, we study the 5s^{2 1}S₀–5s5p ³P₀ transition in In⁺, at a wavelength of 237 nm. This resonance has a linewidth of 0.8 Hz, with systematic frequency shifts expected to be at the mHz level. A fractional resolution of 1.3 × 10⁻¹³ has been so far achieved, limited by the frequency instability of the clock laser used to excite the line. The absolute frequency of the ¹S ₀–³P ₀ transition was measured using the frequency comb of a mode-locked fs laser, and as a reference a methane-stabilized He–Ne laser at 3.4 μm, which was calibrated against an atomic caesium fountain clock. The transition frequency was determined as 1267 402 452 899.92 (0.23) kHz, the uncertainty being limited by the uncertainty of the He–Ne standard. The short-term frequency stability of the clock laser was recently greatly improved. With a new laser set-up a linewidth < 4 Hz (FWHM) for integration times up to 26 s was achieved, using laser platforms which actively isolate from external mechanical vibrations.