1. School of Physics and ElectronicsHunan University Changsha 410082 Hunan P. R. China
2. Synergetic Innovation Center for Quantum Effects and ApplicationKey Laboratory of Low‐dimensional Quantum Structures and Quantum Control of Ministry of EducationSchool of Physics and ElectronicsHunan Normal University Changsha 410081 P. R. China
3. Beijing Institute of Graphic Communication Beijing 102600 P. R. China