Absolute Frequency Measurements of the $D$ Lines in $9{\mathrm{Be}}^{+}$ Using a Single Trapped Ion

D. M. Fairbank, A. L. Banducci, R. W. Gunkelman, J. B. VanArsdale, M. L. Vildibill, and S. M. Brewer, Phys. Rev. Lett. 131, 093001 – Published 1 September 2023

Optical frequencies of the $D$ line transitions in $9{\mathrm{Be}}^{+}$ were measured with a relative uncertainty of $\Delta \nu /\nu =5\times {10}^{-11}$. The results represent the highest accuracy achieved on a broad electric dipole-allowed (E1) transition in a trapped ion experiment to date, enabled in part by detailed consideration of photon recoil and quantum interference. Measurements were made on a single laser-cooled ion stored in a radio frequency Paul trap, using a spectroscopy laser stabilized to an optical frequency comb and referenced to UTC (NIST). The uncertainties in the $D_1$ and $D_2$ lines have been reduced by a factor of 10 and 30, respectively, compared to previous work. We have extracted the $2P$ fine structure splitting, $\Delta {\nu }_{fs}=197064.54\left(7\right)\mathrm{MHz}$, and the ${2P}_{1/2}$ hyperfine constant, $A_{P^{}}=-117.92\left(4\right)\mathrm{MHz}$.