Description
The electric quadrupole transition of the $^{88}$Sr$^+$ ion at 445 THz is one of 11 recommended optical transitions that can be used as secondary representations of the SI second. Progress towards the redefinition of the SI second using an optical clock indicates that a new definition will be based on either one atomic optical transition or an ensemble of such reference transitions, selected from the recommended frequencies. One criteria that must be met before the SI second is redefined is continuity with the current definition of the SI second as realized with cesium fountain clocks, at a level of $3\times10^{-16}$ or better.
Here, we report a new absolute frequency measurement of the $^{88}$Sr$^+$ ion optical transition. The measurement campaign of the ion frequency lasted 12 days in June 2017, and provided 92 hours of comparison data with a local maser flywheel oscillator at NRC. The link between the optical standard and the SI second was completed by measuring the maser continuously during the month of June 2017 with respect to the SI second using a GPS link. The SI second is realized with primary
and secondary frequency standards (PSFS) that report to the BIPM. The maser frequency (UTC(NRC)), averaged over a 30-day period, is provided in the Circular T #354 report published by the BIPM.
The uncertainty of the absolute frequency measurement has three dominant components of comparable values ($2.2$ to $2.5 \times 10^{-16}$): the dead time uncertainty caused by the intermittent operation of the optical clock, the GPS link uncertainty, and the uncertainty of the realization of the SI second with the PSFS. The optical clock evaluated uncertainty of $1.2\times10^{-17}$ is insignificant in comparison. The main improvements compared to previous measurement campaigns are a better evaluation of the dead time uncertainty based on a noise model of maser, and the use of a maser with better noise characteristics than previously available.
The frequency measurement accuracy has a fractional uncertainty of $4.2\times10^{-16}$. This is a factor of four better than our our previous best determination and represents a significant step towards meeting the continuity criterion with the current definition of the SI second.
| Presenter name | Pierre Dubé |
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| How will you attend ICAP-27? | I am planning on in-person attendance |
