**Acknowledgements**

This work was supported by a grant from the Tsinghua University Initiative Scientific Research Program (No. 20131089382) and the National Natural Science Foundation of China (No. 61171002, 60372023). We would like to thank Zexuan Hao and Ziliang Zhang from the Department of Electronic Engineering, Beijing National Research Center for Information Science and Technology (BNRist), Tsinghua University for useful advice during the experiment and manuscript development.
