FREQUENCY COMPARISON OF <font>Al</font>+ AND <font>Hg</font>+ OPTICAL STANDARDS

We compare the frequencies of two single ion frequency standards: Al and Hg . Systematic fractional frequency uncertainties of both standards are below 10, and the statistical measurement uncertainty is below 5&#215; 10. Recent ratio measurements show a reproducibility that is better than 10. Although single-ion optical frequency standards promise a potential accuracy of 10 or better, this long-standing goal has not yet been realized due to various technical difficulties. Here we report progress for the NIST Hg and Al single-ion standards, as their systematic fractional frequency uncertainty approaches 10. In these measurements, the fourth harmonics of two clock lasers are locked to the mercury and aluminum clock transitions at 282 and 267 nm respectively. An octave-spanning self-referenced Ti:Sapphire femtosecond laser frequency comb (FLFC) is phase-locked to one clock laser, and the heterodyne beat-note of the other clock laser with the nearest comb-tooth is measured. The various beat-note and offset frequencies can be combined to yield a frequency ratio, which is independent of the Cs-based definition of the second, allowing this ratio to be measured even more accurately than the fundamental unit of time can be realized. In more recent comparisons of the frequencies of the two clock lasers, an octave-spanning self-referenced fiber comb laser has provided a second independent measure of the frequency ratio.