High-resolution spectroscopy in p115sInp+s and prospects for an indium ion optical frequency reference

Abstract

The indium ion is among the candidates for optical frequency references because of the comparative simplicity of its laser cooling and trapping, the high quality factor (Q =1.5 x 1015) of its clock transition, and the insensitivity of its clock transition to shifts. The indium ion has an alkaline-earth-like spectrum with a cycling transition from the ground state to 3P1 at 230 nm. We report high-resolution laser spectroscopy of the ground-state to 3P 0 "clock" transition at 237 nm in single indium ions confined in a Paul-Straubel trap. A frequency-quadrupled diode laser cools and detects the ion on the 3P1 transition while a frequency-quadrupled Nd:YAG nonplanar ring oscillator (NPRO) at 946 nm stabilized to a vertically-mounted high-finesse optical cavity is tuned to the frequency of the metastable transition. The clock transition was observed using the shelving technique, giving absorption spectra as narrow as 150 Hz with 33% shelving contrast. Systematic shifts of the indium ion clock transition include a linear nuclear Zeeman shift, a blackbody shift estimated as -264(40) mHz, and a second-order Zeeman shift of -40 mHz/G.