Absolute frequency measurement of the optical clock transition in with an uncertainty of using a frequency link to international atomic time

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.