The optical calcium frequency standard

An optical wavelength/frequency standard using the intercombination line ³P₁-¹S₀ of calcium at λ≈657 nm as clock transition is described. An ensemble of cold ⁴⁰Ca atoms was prepared in a magneto-optical trap utilizing the cooling transition ¹P₁-¹S₀ at λ≈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 observed. The frequency of the standard ν_Ca determined against the primary cesium (Cs) atomic clock by a phase-coherent chain is ν_Ca=455 986 240 494.13 kHz with a total relative uncertainty of 2.5×10^-13