1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
2. State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, P.R. China.
3. CNR NANOTEC Institute of Nanotechnology, via Monteroni, 73100 Lecce, Italy.
4. Beijing Academy of Quantum Information Sciences, Beijing 100193, P.R. China.