Absolute frequency measurement of the 171 Yb optical lattice clock at INRIM

We present the absolute frequency measurement of the transition 1 S 0 – 3 P 0 at 578 nm in ytterbium 171 realized in an optical lattice frequency standard relative to the cryogenic caesium fountain ITCsF2. The measurement result is 518 295 836 590 863.59(31) Hz with a relative standard uncertainty of 5.9 × 10 −16 . A contribution of 1.6 × 10 −16 is coming from the ytterbium clock and we are working to reduce this contribution to the 1 × 10 −17 level.