Atomic clocks based on optical transitions, or “optical clocks,” have been suggested as candidates for the next-generation time and frequency standards. Owing to their high operating frequencies, optical clocks promise orders of magnitude improvement in the stability and accuracy over the current standard, cesium fountain clocks. Optical clocks can be categorized by the reference atom, whether it be an individual atom/ion or ensemble of neutral atoms. Currently, the most accurate optical clocks are based on single trapped ions, partially due to the exquisite control of the fields affecting their electronic and motional quantum states. In a recent measurement of the frequency ratio between single-ion optical clocks based on ²⁷Al⁺ and ¹⁹⁹Hg⁺ at NIST, the combined statistical and systematic uncertainties have reached 5.2 ´ 10^-17[1].  To further improve the accuracy and stability of the ²⁷Al⁺ optical clock, we have developed a new trap as well as the lasers that enable the use of ²⁵Mg⁺ for efficient sympathetic cooling of ²⁷Al⁺ and clock-state detection. These developments have reduced the systematic uncertainty below 10^-17. In the new clock apparatus we have demonstrated spectroscopy of the ²⁷Al^{+ 1}S₀ to ³P₀ transition with a quality factor of Q = 4.2 ´ 10¹⁴ and signal contrast approaching unity. The latest comparison between the two ²⁷Al⁺ optical clocks shows that the two clocks agree to within the statistical uncertainty of 2.5 ´ 10^-17 when the known systematic shifts are subtracted. One of the largest relative shifts between the two clocks is the gravitational red shift due to the difference in height of about 17 cm.

[1] T. Rosenband et al., Science 319, 1808 (2008).