Direct comparison of two cold-atom-based optical frequency standards by using a femtosecond-laser comb

With a fiber-broadened, femtosecond-laser frequency comb, the 76-THz interv al between two laser-cooled optical frequency standards was measured with a statistical uncertainty of 2 x 10(-13) in 5 s, to our knowledge the best s hort-term instability thus far reported for an optical frequency measuremen t. One standard is based on the calcium intercombination line at 657 nm, an d the other, on the mercury ion electric-quadrupole transition at 282 nm. B y linking this measurement to the known Ca frequency, we report a new frequ ency value for the Hg+ clock transition with an improvement in accuracy of similar to 10(5) compared with its best previous measurement.