High-accuracy frequency standards using laser-cooled Hg/sup +/ ions

We discuss frequency standards based on laser-cooled /sup 199/Hg/sup +/ ions confined in cryogenic rf (Paul) traps. In one experiment, the frequency of a microwave source is servoed to the ions´ ground-state hyperfine transition at 40.5 GHz. For seven ions and a Ramsey free precession time of 100 s, the fractional frequency stability is 3.3 (2)/spl times/10/sup -13/ /spl tau//sup -1/2/ for measurement times /spl tau/<2 h. The ground-state hyperfine interval is measured to be 40 507 347 996.841 59 (14) (41) Hz, where the first number in parentheses is the uncertainty due to statistics and systematic errors, and the second is the uncertainty in the frequency of the international time scale to which the standard is compared. In a second experiment under development, a strong-binding cryogenic trap will confine a single ion for use in an optical frequency standard based on a narrow electric quadrupole transition at 282 nm.