Observation of the ${}^2{S}_{1/2}{-}^2{F}_{7/2}$ electric octupole transition in a single ${}^{171}{\mathrm{Yb}}^{+}$ ion

The ${}^2{S}_{1/2}{\mathrm{}(F=0\mathrm{})-}^2{F}_{7/2}\mathrm{}(F=3\mathrm{})\mathrm{}$ electric octupole transition in ${}^{171}{\mathrm{Yb}}^{+}$ has been detected by observing quantum jumps in the fluorescence of a single ion held in an electrodynamic trap. The transition frequency has been measured as 642 121 498.1 (0.8) MHz. The extreme weakness of this transition makes it ideally suited as an optical frequency reference of subhertz linewidth. The absolute frequencies of a series of Doppler-free absorptions in tellurium vapor are also reported. One of these absorptions was used as a local frequency reference while locating the electric octupole transition.