Optical frequency standard based on cold Ca atoms

An optical frequency standard ((lambda)=657 nm) based on cold neutral Ca atoms released from a magnetooptical trap has been realized. Systematic contributions to the uncertainty resulting from the residual velocity of the atoms and the acceleration in the gravitational field are determined and reduced by using a combination of different atom interferometers. With the uncertainty for the effect of cold collisions reduced and an AC Stark shift eliminated, the relative uncertainty in the measured frequency of the Ca clock transition is reduced to 2.10^-14. The application of a novel method for producing ultracold atoms showed the potential to further reduce the relative uncertainty to below 10^-15.