[1] Tsinghua Univ, Dept Phys, State Key Lab Low Dimens Quantum Phys, Beijing 100084, Peoples R China
[2] Chinese Acad Sci, Beijing Natl Lab Condensed Matter Phys, Inst Phys, Beijing 100190, Peoples R China
[3] Max Planck Inst Solid State Res, Heisenbergstr 1, D-70569 Stuttgart, Germany
[4] Frontier Sci Ctr Quantum Informat, Beijing 100084, Peoples R China