| Summary: | We reevaluate the blackbody radiation (BBR) induced ac-Stark shift of the strontium clock transition (5s5p)^{3}P_{0}-(5s^{2})^{1}S_{0} at 698nm used as reference in optical lattice clocks and as secondary representation of the SI unit “second.” At room temperature, this frequency shift is on the order of 5×10^{−15} of the transition frequency and causes the largest correction in strontium lattice clocks. With the ongoing reduction of measurement uncertainties of optical clocks, an assessment of the approximations made in the evaluation of the BBR shift is advised. Our reevaluation leads to an increase of the BBR correction by 4×10^{−18} for clock operation at 300K, considerably larger than its present uncertainty of 1.6×10^{−18}. Consistently describing accurately known atomic properties with an atomic structure model, we reduce the fractional uncertainty of the atomic response to 1×10^{−18} at 300K, which allows operating Sr lattice clocks with an uncertainty of 1×10^{−18} at room temperature.
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