Progress on the 40Ca+ ion optical clock
Zhang Baolin1, 2, 3, Huang Yao1, 2, Zhang Huaqing1, 2, 3, Hao Yanmei1, 2, 3, Zeng Mengyan1, 2, 5, Guan Hua1, 2, †, Gao Kelin1, 2, 4, ‡
       

Frequency difference between two 40Ca+ ion optical clocks.