Strontium optical lattice clock at the National Time Service Center
Wang Ye-Bing1, 2, Yin Mo-Juan1, Ren Jie1, Xu Qin-Fang1, Lu Ben-Quan1, 2, Han Jian-Xin1, 2, Guo Yang1, 2, Chang Hong1, †
       

(color online) Energy-levels schematic of a 87Sr atom.