The progress of ⁴⁰Ca⁺ optical frequency standards

The calcium ion optical frequency standard is one of a variety of different kinds of optical frequency standards. Innovation Academy for Precision Measurement Science and Technology (formerly Wuhan Institute of Physics and Mathematics), Chinese Academy of Sciences has selected calcium ions as the optical frequency standard reference since 2000, and has successively step by step solved a series of problems, such as the trapping and laser cooling of singleions, the frequency stabilization to achieve ultra-narrow linewidth lasers, and especially the suppression of the systematic uncertainty due to the micromotion and the blackbody radiation. The overall systematic uncertainty of the liquid nitrogen optical frequency standard and the room temperature optical frequency standard were evaluated as 3.0×10^-18 and 4.8×10^-18 respectively, and the stability was measured as 6.3×10^-18 at 520000 s. A transportable and robust calcium optical clock has been built with the integrated design and the uncertainty of the transportable clock was evaluated as 1.3×10^-17. and a preliminary geopotential measurement was made with the transportable clock. The uncertainty for the absolute frequency measurement has been improved to 3.2×10^-16, referenced to both the local cesium fountain clock and the local UTC(k) referenced to the SI second. The calcium ion optical clock transition has been recommended by the International Bureau of Weights and Measures (BIPM) as a new reference for the secondary representation of the SI second.