A Ca optical frequency standard based on ultra-cold atoms

With recently developed strategies to reduce frequency offsets due to residual first-order Doppler effects the relative uncertainty of the Ca optical frequency standard at PTB was reduced to 2.7-10/sup -14/. The weighted mean of the optical frequency measured by a harmonic frequency chain and recently by a femtosecond laser comb is (455 986 240 494 154 /spl plusmn/ 13) Hz. The use of ultra-cold atoms prepared by a novel two-stage cooling method will allow to further reduce frequency offsets.