Radium Ion Optical Clock
C. A. Holliman, Mingyu Fan, Alisha A. Contractor, Samuel M. Brewer, Andrew M. Jayich
Abstract
We report the first operation of a ${\mathrm{Ra}}^{+}$ optical clock, a promising high-performance clock candidate. The clock uses a single trapped $^{226}{\mathrm{Ra}}^{+}$ ion and operates on the $7s\text{ }^{2}{S}_{1/2}\ensuremath{\rightarrow}6d\text{ }^{2}{D}_{5/2}$ electric quadrupole transition. By self-referencing three pairs of symmetric Zeeman transitions, we demonstrate a frequency instability of $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}/\sqrt{\ensuremath{\tau}}$, where $\ensuremath{\tau}$ is the averaging time in seconds. The total systematic uncertainty is evaluated to be $\mathrm{\ensuremath{\Delta}}\ensuremath{\nu}/\ensuremath{\nu}=9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$. Using the clock, we realize the first measurement of the ratio of the ${D}_{5/2}$ state to the ${S}_{1/2}$ state Land\'e $g$-factors: ${g}_{D}/{g}_{S}=0.598\text{ }805\text{ }3(11)$. A ${\mathrm{Ra}}^{+}$ optical clock could improve limits on the time variation of the fine structure constant, $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{\ensuremath{\alpha}}/\ensuremath{\alpha}$, in an optical frequency comparison. The ion also has several features that make it a suitable system for a transportable optical clock.