Abstract
The highly forbidden electric octupole transition in
is a potential candidate for a redefinition of the SI second. We present a measurement of the absolute frequency of this optical transition, performed using a frequency link to International Atomic Time to provide traceability to the SI second. The
optical frequency standard was operated for 76% of a 25-day period, with the absolute frequency measured to be 642 121 496 772 645.14(26) Hz. The fractional uncertainty of
is comparable to that of the best previously reported measurement, which was made by a direct comparison to local caesium primary frequency standards.
Acknowledgements
We thank Peter Whibberley for helpful discussions and E. Anne Curtis for critical review of the manuscript prior to submission. We also note that our absolute frequency measurement derives its accuracy from the primary standards operated at other national measurement institutes around the world.
Notes
No potential conflict of interest was reported by the authors.