1. Berlin Joint EPR Lab, Fachbereich Physik, Freie Universität Berlin, D-14195 Berlin, Germany
2. National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China
3. Synergetic Innovation Center in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China