An optical frequency standard with ultracold calcium atoms

An optical frequency standard is operated at PTB where the frequency of a laser is stabilized to the inter- combination transition (lambdaap 657 nm) in laser cooled Ca atoms. Previously the uncertainty resulting from the residual velocity of the cold atoms at 3 mK limited the fractional uncertainty to 2 times 10^-14. We now apply a novel method for producing ultra-cold atoms (T ap 10 muK). The frequency of the intercombination line has been measured with respect to the Cs hyperfine transition over eight years and a first frequency measurement with ultracold atoms has now been performed where the contributions from the residual velocity become insignificant at that level of uncertainty. A fractional uncertainty below 10^-15 is expected for the near future.