⁴⁰Ca⁺ optical clocks in China

In this review, the improvements made in the past two decades for ⁴⁰Ca⁺ optical clocks in China are presented. Uncertainty of below $1.3×10−17$ was achieved by applying the “magic” RF trapping frequency and introducing a generalized Ramsey excitation scheme. The stability of ⁴⁰Ca⁺ optical clocks has been improved to $2.5×10−15/τ$ with an uptime rate of 93.8% by implementing two ameliorated lock algorithms in the Ramsey excitation scheme. A long-term clock frequency comparison shows a clock stability of 6.3 × 10⁻¹⁸ in an averaging time of 524 000 s. A robust and transportable clock installed in an air-conditioned car trailer can achieve almost the same performance of laboratory clocks. A height difference between transportable and laboratory clocks was measured with an uncertainty of 0.33 m and the absolute frequency of ⁴⁰Ca⁺ optical clock transitions was remeasured as 411 042 129 776 400.41(23) Hz, with a fractional uncertainty of $5.6×10−16$ based on a Cs fountain clock in the National Institute of Metrology after the transportable clock was transported from Wuhan to Beijing. The author predicts that transportable single-ion optical clocks especially based on the ⁴⁰Ca⁺ will make a significant contribution to the construction of a world-wide optical clock network and the redefinition of the unit of time in the future.