1 Ministry of Education Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Shaanxi Province Key Laboratory of Quantum Information and Quantum Optoelectronic Devices, School of Physics, Xi’an Jiaotong University, Xi’an 710049, China
2 School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
3 Key Laboratory of Time and Frequency Primary Standards, National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China
4 Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, H-1525 Budapest, Hungary
5 e-mail: mingtaocao@ntsc.ac.cn