Improved systematic evaluation of a strontium optical clock with uncertainty below 1 × 10 − 18

Abstract

We report a systematic uncertainty of 9.2 × 10 − 19 for the Sr1 optical lattice clock at the University of Science and Technology of China (USTC), achieving accuracy at the level required for the roadmap of the redefinition of the SI second. A finite-element model with in situ -validated, spatially-resolved chamber emissivity reduced blackbody radiation (BBR) shift uncertainty to 6.3 × 10 − 19 . Concurrently, the externally mounted lattice cavity, by providing a larger beam waist, reduced the atomic density and thereby suppressed the density shift. Enhanced lattice depth modulation consolidated lattice light shift uncertainty to 6.3 × 10 − 19 by enabling simultaneous determination of key polarizabilities and magic wavelength. Magnetic shifts were resolved below 10 −18 via precision characterization of the second-order Zeeman coefficient. Supported by a clock laser stabilized on an ultralow-expansion glass cavity with crystalline-coated mirrors and refined temperature control suppressing BBR fluctuations, the clock also achieves a frequency stability better than 1 × 10 − 18 at 30 000 s averaging time. These developments collectively establish a new benchmark in USTC Sr1 clock performance and pave the way for high-accuracy applications in metrology and fundamental physics.