All-optical atomic clocks

We have developed two all-optical clocks in which the rf output is generated directly from the optical process within the clock-work. The frequency comb created by a femtosecond mode-locked laser and a microstructure fiber is used to phase-coherently measure the frequencies of bot the Hg+ and Ca optical frequency standards with respect to the SI second as realized at NIST. 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. This work begins to reveal the high stability and potential for accuracy of optical atomic clocks. Furthermore, when combined with previous measurements, we find no time variations of these atomic frequencies within the uncertainties of \(partial derivativef(Hg)/partial derivativet)/f(Hg)\ = 2 x 10(-14) yr(-1) and \(partial derivativef(Ca)/partial derivativet)/f(Ca)\ = 8 x 10(-14) yr(-1).