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 as realized at NIST. We find the transition frequencies to be fHg=1 064 721 609 899 143(10) Hz and fCa=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 |(dfCa/dt)/fCa| < 8 x 10-14 yr-1, and |(dfHg/dt)/fHg|< 30 x 10-14 yr-1.
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