With a fiber-broadened, femtosecond-laser frequency comb, the 76-THz interval between two laser-cooled optical frequency standards was measured with a statistical uncertainty of 2×10<sup>-13</sup> in 5 s , to our knowledge the best short-term instability thus far reported for an optical frequency measurement. One standard is based on the calcium intercombination line at 657 nm, and the other, on the mercury ion electric-quadrupole transition at 282 nm. By linking this measurement to the known Ca frequency, we report a new frequency value for the Hg<sup>+</sup> clock transition with an improvement in accuracy of ~10<sup>5</sup> compared with its best previous measurement.
© 2001 Optical Society of America
K. R. Vogel, S. A. Diddams, C. W. Oates, E. A. Curtis, R. J. Rafac, W. M. Itano, J. C. Bergquist, R. W. Fox, W. D. Lee, J. S. Wells, and L. Hollberg, "Direct comparison of two cold-atom-based optical frequency standards by using a femtosecond-laser comb," Opt. Lett. 26, 102-104 (2001)