IT-Yb1 Optical Lattice Clock: Absolute Frequency Measurement at the Cs Fountain Uncertainty Level

We report the result of the new absolute frequency measurement of 171Yb performed at INRIM. The frequency of our optical lattice clock IT-Yb1 is measured against the local cryogenic Cs fountain IT-CsF2 during a campaign that lasted 14 months. The frequency measurements were performed with two different techniques, i.e. using a hydrogen maser as a transfer oscillator or by synthesizing a low-noise microwave for the Cs interrogation from the Yb ultrastable laser with an optical frequency comb. The frequency of the Yb unperturbed clock transition is 518 295 836 590 863.44(14) Hz, with a total fractional uncertainty of 2.7 × 10-16, limited by the uncertainty of IT-CsF2. This is the absolute frequency measurement of a Yb clock against a Cs fountain with the lowest uncertainty. The result agrees with the Yb frequency value recommended by the Consultative Committee for Time and Frequency (CCTF). Moreover, the Yb data acquired during this campaign have been submitted to the Bureau of Weights and Measures (BIPM) to contribute to the calibration of the International Atomic Time (TAI). This work confirms the reliability of Yb as a secondary representation of the second.