An Aluminum Ion Optical Clock Using Quantum Logic

The 267 nm $^1$S$_0 \rightarrow ^3$P$_0$ transition in $^{27}$Al$^+$ combines several attractive characteristics as an atomic reference for an optical clock with high stability and accuracy. Its sharp clock transition (7 mHz natural linewidth) has a very small electric quadrupole moment, a low quadratic Zeeman coefficient (0.7 Hz/gauss$^2$), as well as a small room temperature blackbody shift ($\Delta\nu/\nu < 10^{-17})$. We have used quantum logic based spectroscopy$^{a,b}$ to operate an Al$^+$ optical frequency standard in which a stable laser oscillator at 534 nm is doubled and locked to the Al$^+$ $^1$S$_0 \rightarrow ^3$P$_0$ transition. The frequency of this optical standard was compared to the NIST $^{199}$Hg$^+$ optical frequency standard using a femtosecond frequency comb, resulting in a frequency ratio measurement with $\Delta\nu/\nu < 10^{-16}$ statistical uncertainty. The systematic uncertainty in the Al$^+$ clock frequency has a similar magnitude, and is dominated by second order Doppler shifts due to secular motion and micromotion. \newline [a] D. J. Wineland \emph{et al.}, Proc. 6th Symp. on Freq. Standards and Metrology, 361 (2002) \newline [b] P. O. Schmidt \emph{et al.}, Science \textbf{309}, 749 (2005)