A strontium lattice clock with both stability and uncertainty below 5×10−18

We report the realization of the closed-loop operation of an optical lattice clock based on 87Sr atoms. A cavity-stabilized 698 nm laser is used to probe the 1S0→3P0 clock transition of strontium atoms trapped in optical lattices. Therein, we obtain a Fourier-limited Rabi spectrum with 0.6 Hz linewidth. The two transitions from mF=±9/2 ground states are alternatively interrogated to realize the closed-loop operation of the clock, and the clock laser light is frequency-stabilized to the center of the two transitions. Based on the interleaved measurement, the frequency instability of a single optical clock is optimized for the Dick effect, which is demonstrated to be 4.5×10−16/τ , with τ being the averaging time for measurement. Further, we build another similar setup of the strontium lattice clock, which is used for the asynchronous comparison between the two clocks, where the stability is measured as 2.1×10−18 at 47 000 s. Moreover, we carefully calibrate the systematic effects of the Sr1 optical clock, and the total uncertainty is evaluated as 4.4×10−18 .