An optical frequency standard with cold calcium atoms

We report on the current status and the prospects of the calcium optical frequency standard which is based on a ballistically expanding cloud of laser cooled Ca atoms. The optical frequency of the standard at 456 THz (657 nm) has been realized and measured by comparison with the primary Cs atomic clock with a fractional uncertainty of 2 10 -14 . The combination of different atom interferometric techniques allows to investigate and to reduce the largest systematic contribution to the uncertainty resulting from the velocity of the freely falling lasercooled atoms. It is shown how the application of a novel method for producing ultracold atoms can reduce the fractional uncertainty further to 10 -15 .