title: Absolute frequency measurements of the Hg+ and Ca optical clock transitions with a femtosecond laser.
abstract: The frequency comb created by a femtosecond mode-locked laser and a microstructured fiber is used to phase coherently measure the frequencies of both the Hg+ and Ca optical standards with respect to the SI second. We find the transition frequencies to be f(Hg) = 1 064 721 609 899 143(10) Hz and f(Ca) = 455 986 240 494 158(26) Hz, respectively. In addition to the unprecedented precision demonstrated here, this work is the precursor to all-optical atomic clocks based on the Hg+ and Ca standards. Furthermore, when combined with previous measurements, we find no time variations of these atomic frequencies within the uncertainties of the absolute value of( partial differential f(Ca)/ partial differential t)/f(Ca) < or =8 x 10(-14) yr(-1) and the absolute value of(partial differential f(Hg)/ partial differential t)/f(Hg) < or =30 x 10(-14) yr(-1).
authors.affiliation: Time and Frequency Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA.
authors.forename: T
authors.initials: T
authors.lastname: Udem
authors.forename: S A
authors.initials: SA
authors.lastname: Diddams
authors.forename: K R
authors.initials: KR
authors.lastname: Vogel
authors.forename: C W
authors.initials: CW
authors.lastname: Oates
authors.forename: E A
authors.initials: EA
authors.lastname: Curtis
authors.forename: W D
authors.initials: WD
authors.lastname: Lee
authors.forename: W M
authors.initials: WM
authors.lastname: Itano
authors.forename: R E
authors.initials: RE
authors.lastname: Drullinger
authors.forename: J C
authors.initials: JC
authors.lastname: Bergquist
authors.forename: L
authors.initials: L
authors.lastname: Hollberg