Recommendation 1 of the 21st CCTF (2017)

Recommendations for operating, comparing and reporting frequency standards as secondary representations of the second in preparation for a redefinition of the second by optical transitions

The Consultative Committee for Time and Frequency (CCTF), at its 21st session in 2017,

considering that

• a list of secondary representations of the second (SRS) has been maintained following the recommendations of the CIPM,


• different optical SRS have estimated fractional frequency uncertainties nearly two orders of magnitude lower than those of the best caesium primary standards,


• improvements in uncertainty associated with optical frequency standards are ongoing,


• a roadmap for a future redefinition of the second using optical frequency standards has been agreed by the CCTF;

recommends that

• the institutes put effort into operating their frequency standards to realize SRS in such a way that they routinely contribute to TAI via reporting to the BIPM,


• the optical standards be compared with uncertainties that are comparable to the estimated uncertainties of the standards themselves,


• the institutes measure the frequencies of the realizations of their SRS with respect to the best primary caesium standards as a necessary requirement for a possible future redefinition of the second in terms of optical transitions,


• the relevant CCTF working groups finalize the milestones for a redefinition and regularly inform the CIPM about the progress towards meeting these milestones.

DOI : 10.59161/CCTF2017REC1E

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