An optical clock based on a single trapped Hg-199(+) ion

Microwave atomic clocks have been the de facto standards for precision time and frequency metrology over the past 50 years, finding widespread use in basic scientific studies, communications, and navigation. However, with its higher operating frequency, an atomic clock based on an optical transition can be much more stable. We demonstrate an all-optical atomic clock refere nced to the 1.064-petahertz transition of a single trapped Hg-199(+) ion. A clock-work based on a mode-locked femtosecond laser provides output pulses at a 1-gigahertz rate that are phase-coherently locked to the optical freq uency. By comparison to a laser-cooled calcium optical standard, an upper l imit for the fractional frequency instability of 7 x 10(-15) is measured in 1 second of averaging-a value substantially better than that of the world' s best microwave atomic clocks.