1. Tsinghua University 1 Tsinghua National Laboratory for Information Science and Technology, , Beijing 100084, China and Department of Automation, , Beijing 100084, China
2. AMSS 2 Institute of Mathematics, , Academia Sinica, Beijing 100080, China
3. Hong Kong Baptist University 3 Department of Physics, , Kowloon Tong, Hong Kong
4. Potsdam Universität 4 Institut für Physik, , Am Neuen Palais 10, D-14469, Germany