Experimental Progress on the NIST $^{27}$Al$^{+}$ Optical Clock

A recent measurement of the frequency ratio between single-ion optical clocks based on $^{27}$Al$^{+}$ and $^{199}$Hg$^{+}$ at NIST showed a combined statistical and systematic uncertainty of 5.2 $\times $ 10$^{-17}$[1]. Here we report progress on improving both the accuracy and stability of the $^{27}$Al$^{+}$ optical clock. We have developed a new trap and laser systems that enable the use of $^{25}$Mg$^{+}$ for sympathetic cooling and clock-state detection of $^{27}$Al$^{+}$. These developments should reduce time-dilation shifts caused by harmonic motion of the ions and thus lower the dominant systematic uncertainty below 10$^{-17}$. In the new clock apparatus we have demonstrated spectroscopy of the $^{27}$Al$^{+}$ $^{1}$S$_{0}$ to $^{3}$P$_{0}$ transition with a quality factor of Q = 3.5 $\times $ 10$^{14}$ and simultaneously a contrast approaching unity. In addition, we have developed techniques for the sympathetic laser cooling and quantum logic spectroscopy of multiple aluminum ions with the goal of further improving measurement stability [2]. *supported by ONR and NIST [1] T. Rosenband et al., Science \textbf{319}, 1808 (2008) [2] D. B. Hume et al., Phys. Rev. Lett. \textbf{99}, 120502 (2007)