Precision optical frequency measurements and coherent spectroscopy with a single trapped ion standard

摘要

A single ion, trapped, laser cooled and probed on an ultra-narrow transition, provides what is widely considered the best approximation to an ideal, isolated optical frequency reference. Our group has been actively studying the 445-THz (674 nm), 5s~2S_(1/2)-4d~2D_(5/2) transition in ~(88)Sr~+. Individual Zeeman component linewidths of 250 Hz were observed and a probe laser system was locked to the center frequency of the Zeeman spectrum. A cesium-based frequency chain was used to measure the center frequency of the ~(88)Sr~+ S-D spectrum to an accuracy of 200 Hz. Under our current experimental conditions, the magnitudes of the systematic shifts in the linecenter position are estimated to be less than 1 part in 10~(15). As part of our efforts in improving the trapped ion standard, we have studied the coherent excitation of a single ion via Rabi pulse and Ramsey fringe interrogation. Initial results yielding Ramsey fringe widths down to 840 Hz were obtained, and some decoherence and motional properties of the ion system were investigated. The Sr~+ standard was applied recently to the well-known He-Ne/I_2 standard at 633 nm. This measurement provided an accuracy comparison between a traditional frequency chain and a femtosecond laser frequency comb. In addition, after several months and transportation over a significant portion of the globe. ~(88)Sr~+ calibrated 633 nm lasers from NRC and BIPM have shown agreement to 1 kHz in their originally determined absolute optical frequencies. The results of these intercomparisons point to potential worldwide accuracy improvement of working 633 nm radiation standards.