Absolute frequency measurement of a Yb optical clock at the limit of the Cs fountain

We present the new absolute frequency measurement of ytterbium ( 171 Yb) obtained at INRiM with the optical lattice clock IT-Yb1 against the cryogenic caesium ( 133 Cs) fountain IT-CsF2, evaluated through a measurement campaign that lasted 14 months. Measurements are performed by either using a hydrogen maser as a transfer oscillator or by synthesizing a low-noise microwave for Cs interrogation using an optical frequency comb. The frequency of the 171 Yb unperturbed clock transition 1 S 0 → 3 P 0 results to be 518 295 836 590 863.44(14) Hz, with a total fractional uncertainty of 2.7 × 10 − 16 that is limited by the uncertainty of IT-CsF2. Our measurement is in agreement with the Yb frequency recommended by the Consultative Committee for Time and Frequency. This result confirms the reliability of Yb as a secondary representation of the second and is relevant to the process of redefining the second in the International System of Units on an optical transition.