Optical frequency standards (OFS) exploits as reference the frequency of an atomic transition that is strongly forbidden by quantum rules. Using in particular Neutral Alkaline-Earth-Like (AEL), the metastable triplet in fermionic isotopes offers an outstanding reference with the 1S0-3P0 transition, weakly allowed by the hyperfine mixing of the triplet, with a resulting natural linewidth of about 10 mHz.
In OFS, the accuracy of the realization with respect to the unperturbed atom condition is the highest, outperforming also the best atomic frequency standards based on Caesium. Different experiments report an accuracy as high as few parts in 1e18. For this reasons, OFS are now an outstanding candidate for the redefinition of the unit of time.
Moreover, OFS are precise quantum sensors, with unique features, for example in the direct sensing of gravity potential. In fact, according to the law of General Relativity, two remote clocks experiencing a different gravity potential will have a frequency offset among them. On the Earth, the resulting sensitivity is 1e-16 (relative frequency) per meter. Present OFS accuracy enables to detect a difference of few centimetres: this has triggered a new geodesy, the so culled relativistic or chronometric geodesy.
In Italy, at the Italian Metrological Institute INRIM, a candidate for the redefinition of the secondi s under operation, based on the isotope 171 of Ytterbium, with the clock transition 1S0-3P0 at 578 nm. The absolute frequency of the transition has been measured towards a primary frequency standard based on a cryogenic Cs fountain. The result has been taken into account in June 2017 for the recommendation of 171Yb clock transition as a so called Secondary Representation of the second.
INRIM Yb OFS has an instability of 3e-15 at 1 second and an overall accuracy of e-16, currently under re-evaluation to reach an accuracy of few parts in 1e17. A second Yb clock is under development, aiming at an accuracy of parts in 1e18, coping with relevant limitation coming from the black-body radiation and the shifts of the transition induced by the lattice laser radiation.
At the conference, we will present our recent results in the absolute frequency evaluation of the Yb clock transition, the accuracy evaluation and the status of the second Yb clock.
The roadmap towards the redefinition of the second will be described also.
Finally, we will describe the proof of principle of a relativistic geodesy experiment performed between INRIM (Torino) and Mont Frejus, using an optical fibre link.