Evaluation of a ⁸⁸Sr⁺ Optical Clock with a Direct Measurement of the Blackbody Radiation Shift and Determination of the Clock Frequency

We report on an evaluation of an optical clock that uses the S-2(1/2) -> D-2(5/2) transition of a single Sr-88(+) ion as the reference. In contrast to previous work, we estimate the effective temperature of the blackbody radiation that shifts the reference transition directly during operation from the corresponding frequency shift and the well-characterized sensitivity to thermal radiation. We measure the clock output frequency against an independent Yb-171(+) ion clock, based on the S-2(1/2) (F = 0) -> F-2 (7/2) (F = 3) electric octupole (E3) transition, and determine the frequency ratio with a total fractional uncertainty of 2.3 x 10(-17). Relying on a previous measurement of the Yb-171+ (E3) clock frequency, we find the absolute frequency of the Sr-88(+) clock transition to be 444 779 044 095 485.277(59) Hz. Our result reduces the uncertainty by a factor of 3 compared with the previously most accurate measurement and may help to resolve so far inconsistent determinations of this value. We also show that for three simultaneously interrogated Sr-88(+) ions, the increased number causes the expected improvement of the short-term frequency instability of the optical clock without degrading its systematic uncertainty.