Absolute frequency measurement of a single-ion optical transition: the 24-THz excited-state transition in Ba+

Abstract

A single, laser cooled ion held in a radio-frequency trap is a near ideal system for high resolution spectroscopy and for new optical frequency standards. We report the first absolute frequency measurement of a single ion optical transition using a frequency chain referenced to a primary frequency standard. The D-D excited state, fine structure transition at 24 THz of a single, laser cooled ion was probed using an electro-optic shifted, frequency sideband from an optically pumped ammonia laser operating on the sP (8,6) line. By recording the ion quantum jump rate as a function of laser detuning, single ion lineshapes were obtained free of any Doppler broadening. Measurements of the ion motional temperature gave estimates below 15 mK and a time dilation shift below 2 × 1017. A phase-locked, optical frequency synthesis chain starting from a primary cesium standard to the 24.029 THz ammonia probe laser frequency has been constructed and can provide accuracies of 1013. Simultaneous measurement of the ammonia probe laser frequency and the shifted sideband frequency have enabled an absolute frequency determination with an uncertainty below the kilohertz level at the optical probe frequency of 24 THz limited by the current linewidth and stability of the ammonia probe laser.