40Ca+ optical clocks in China.

In this review, the improvements made in the past two decades for 40Caþ optical clocks in China are presented. Uncertainty of below 1:3 x 10-17 was achieved by applying the "magic" RF trapping frequency and introducing a generalized Ramsey excitation scheme. The stability of 40Caþ optical clocks has been improved to 2:5 x 10-15= ffiffiffi s p 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 6.3x10-18 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 40Caþ optical clock transitions was remeasured as 411 042 129 776 400.41(23) Hz, with a fractional uncertainty of 5:6 x 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 40Caþ 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. [ABSTRACT FROM AUTHOR]