Optical Clocks with Cold Atoms and Stable Lasers*

The performance and prospects for neutral-atom optical frequency standards are discussed based on our recent progress with a calcium optical frequency star~dard. Second stage narrow-line cooling to microkelvin (and even 300 nK) temperatures, combined with launched atoms, should reduce Doppler frequency errors to about 1x10 1 6 . Advanced femtosecond optical frequency combs allow direct comparisons between the Ca optical standard, the Hg* optical standard and the Cs primary standard. These comparisons provide independent "reality checks" on both the stability and accuracy. Relative frequency measurements also constrain the possible time variation of atomic energy levels and fundamental constants.