First phase-coherent frequency measurement of visible radiation

We have determined the frequency of the P-3(1)-S-1(0) intercombination transition of atomic Ca-40 stared in a magneto-optical trap to be nu = 455 986 240493.95 kHz with an estimated standard uncertainty of 0.43 kHz (delta nu/nu < 10(-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.