title: Direct comparison of two cold-atom-based optical frequency standards by using a femtosecond-laser comb.
abstract: 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 2x10(-13) 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(+) clock transition with an improvement in accuracy of ~10(5) compared with its best previous measurement.
authors.forename: K R
authors.initials: KR
authors.lastname: Vogel
authors.forename: S A
authors.initials: SA
authors.lastname: Diddams
authors.forename: C W
authors.initials: CW
authors.lastname: Oates
authors.forename: E A
authors.initials: EA
authors.lastname: Curtis
authors.forename: R J
authors.initials: RJ
authors.lastname: Rafac
authors.forename: W M
authors.initials: WM
authors.lastname: Itano
authors.forename: J C
authors.initials: JC
authors.lastname: Bergquist
authors.forename: R W
authors.initials: RW
authors.lastname: Fox
authors.forename: W D
authors.initials: WD
authors.lastname: Lee
authors.forename: J S
authors.initials: JS
authors.lastname: Wells
authors.forename: L
authors.initials: L
authors.lastname: Hollberg