Absolute frequency measurement of the – transition of 171Yb

We report the absolute frequency measurement of the unperturbed transition – at 578 nm in 171Yb realized in an optical lattice frequency standard relative to a cryogenic caesium fountain. The measurement result is 518 295 836 590 863.59(31) Hz with a relative standard uncertainty of . This value is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the International System of Units.