Absolute Frequency Measurements of the Hg+ and Ca Optical Clock Transitions with a Femtosecond Laser
Th. Udem, S. A. Diddams, K. R. Vogel, C. W. Oates, E. A. Curtis, W. D. Lee, W. M. Itano, R. E. Drullinger, J. C. Bergquist, L. Hollberg
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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