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

TL;DR

This paper reports a highly precise absolute frequency measurement of a Yb optical clock, confirming its suitability as a secondary standard for the second and supporting SI redefinition efforts.

Contribution

It provides the most accurate measurement of the Yb clock transition frequency against a Cs fountain, demonstrating Yb's reliability for time standards.

Findings

Frequency measured at 518,295,836,590,863.44 Hz

Total fractional uncertainty of 2.7e-16

Agreement with CCTF recommended values

Abstract

We present the new absolute frequency measurement of ytterbium (171Yb) obtained at INRiM with the optical lattice clock IT-Yb1 against the cryogenic caesium (133Cs) 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 171Yb unperturbed clock transition 1S0 - 3P0 results to be 518 295 836 590 863.44(14) Hz, with a total fractional uncertainty of 2.7e-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 (CCTF). 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 (SI) on an optical transition