1. Department of Computer Science, Jinan University, Guangzhou 510632, P. R. China
2. State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China
3. School of Physics and Astronomy, Sun Yat-Sen University, Guangzhou 510275, P. R. China
4. Guangxi Key Laboratory of Trusted Software, Guilin University of Electronic Technology, Guilin, P. R. China
5. Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX, USA