Optical frequency standard based on a single 40Ca+

Research on the development of the optical frequency standard based on trapped and cold ⁴⁰Ca⁺ with the 4s ²S_1/2-3d ²D_5/2 clock transition at 729 nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729 nm laser was reduced to less than 10 Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10^?16. The absolute frequency of the clock transition was measured at 10^?15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6) Hz after the correction of the systematic shifts.