Absolute frequency measurements of the Hg+ and Ca optical clock transitions with a femtosecond laser

• Published in: Physical review letters Vol. 86; no. 22; pp. 4996 - 4999
• Main Authors: Udem, T, Diddams, S A, Vogel, K R, Oates, C W, Curtis, E A, Lee, W D, Itano, W M, Drullinger, R E, Bergquist, J C, Hollberg, L
• Format: Journal Article
• Language: English
• Published: United States 28-05-2001
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.86.4996

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).