The optical calcium frequency standard

An optical wavelength/frequency standard using the intercombination line P- 3(1) - S-1(0) of calcium at lambda approximate to 657 mm as clock transitio n is described. An ensemble of cold Ca-40 atoms was prepared in a magneto-o ptical trap utilizing the cooling transition P-1(1) - S-1(0) at lambda appr oximate to 423 nm. The reference signal for the frequency stabilization was generated by time separated field excitation of the clock transition. Sub- kHz resolution close to the natural linewidth of the transition was observe d. The frequency of the standard upsilon(Ca) determined against the primary cesium (Cs) atomic clock by a phase-coherent chain is upsilon(Ca) = 455 98 6 240 494.13 kHz with a total relative uncertainty of 2.5 x 10(-13).