First Phase-Coherent Frequency Measurement of Visible Radiation

Abstract

We have determined the frequency of the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$- ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ intercombination transition of atomic ${}^{40}\mathrm{Ca}$ stored in a magneto-optical trap to be $\nu =455\mathrm{}986240493.95\mathrm{kHz}$ with an estimated standard uncertainty of $0.43\mathrm{kHz}$ $(\delta \nu /\nu <{10}^{-12\mathrm{}})$ using a phase-coherent optical frequency chain from the Cs atomic clock to the visible. This allows the realization of the SI-unit meter according to its definition by visible radiation with 25-fold reduced uncertainty compared to previous measurements.