Ytterbium optical lattice clock with instability of order 10⁻¹⁸

We have built an ytterbium optical lattice clock with improvements over our previous version. An in-vacuum blackbody radiation (BBR) shield is employed to provide a well characterized BBR environment. The effective temperature felt by the atoms can be determined at an accuracy level of 13 mK, leading to a total BBR frequency shift uncertainty of 9.5 × 10⁻¹⁹. We have also built an ultra-stable optical cavity system to pre-stabilize the clock laser, achieving a flicker frequency instability of ∼3 × 10⁻¹⁶. Rabi spectroscopy of the lattice-trapped atoms can achieve sub-Hertz linewidth spectra. Two ytterbium clocks have been operated in an antisynchronized configuration, with real-time BBR-Stark-shift corrections applied to both of them. By comparing the two clocks, we demonstrate a single-clock instability of 5.4 × 10⁻¹⁸ in 4500 s. This clock will be applied for frequency comparisons to other optical clocks of different atomic species in the future.