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 $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}455986240493.95\mathrm{kHz}$ with an estimated standard uncertainty of $0.43\mathrm{kHz}$ $(\ensuremath{\delta}\ensuremath{\nu}/\ensuremath{\nu}<{10}^{\ensuremath{-}12})$ 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.