Abstract

The fluorescence of a single Ba⁺ ion, which is confined in an rf trap and laser cooled, is excited by 493-nm light and 650-nm light, corresponding to the ²S_1/2–²P_1/2 and ²D_3/2–²P_1/2 transitions, respectively. It shows a null, or “dark line” at the S_1/2–D_3/2 resonance of the frequency difference^[1,2], since optical pumping makes a coherent superposition of these low-lying states emerge, which decouples from the light fields: a “trapped” state.^[3] A spectroscopic study of this dark line includes the line’s dependence on those parameters which determine its shape and shifts, and its suitability for a frequency standard: (1) Zeeman-split spectra have been recorded over a broad range of values of the external magnetic field, and solutions of the eight-sublevel optical Bloch equations have been fitted to them (Fig. 1). (ii) The narrowest dark line has been measured. Its width is 75 kHz and results from the decay rate γ_Sd = 12 kHz of the S–D coherence due to laser instabilities and stray magnetic fields and by residual power broadening, (iii) The dark line is unshifted by the light. However, the adjacent “bright” line - the resonance of the S–D coherent superposition with maximum coupling to the light - displays significant Raman light shift, which has been measured.

© 1992 IQEC