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We have measured the absolute frequency of the optical lattice clock based on$^{87}$Sr at PTB with an uncertainty of $3.9\times 10^{-16}$ using two caesiumfountain clocks. This is close to the accuracy of today's best realizations ofthe SI second. The absolute frequency of the 5s$^2$ $^1$S$_0$-5s5p $^3$P$_0$transition in $^{87}$Sr is 429,228,004,229,873.13(17) Hz. Our result is inexcellent agreement with recent measurements performed in differentlaboratories worldwide. We improved the total systematic uncertainty of our Srfrequency standard by a factor of five and reach $3\times 10^{-17}$, openingnew prospects for frequency ratio measurements between optical clocks forfundamental research, geodesy, or optical clock evaluation.

A strontium lattice clock with $3 \times 10^{-17}$ inaccuracy and its frequency