LASER-COOLED MERCURY ION FREQUENCY STANDARD

A stable and accurate frequency standard based on the 40.5 GHz ground- state hyperfine transition in Hg-199(+) ions is described. The ions ar e confined in a cryogenic linear Paul (rf) trap and laser cooled to fo rm a linear crystal. With seven ions and a Ramsey interrogation time o f 100 s, the fractional frequency stability is 3.3 (2) x 10(-13)tau(-1 /2) for measurement times tau < 2 h. The ground-state hyperfine interv al is measured to be 40 507 347 996.841 59 (14) (41) Hz, where the fir st number in the parentheses is the uncertainty due to statistical and systematic effects, and the second is the uncertainty in the frequenc y of the time scale to which our standard is compared.