Acknowledgment. Research was partially sponsored by NSF, DARPA, NIH and NSFC. Q.K. thanks Xiamen University for the financial support during his stay at Georgia Institute of Technology.

• 鸣谢：研究的一部分资金是由美国国家科学基金会，美国国防先期研究计划局，国家卫生机构和国家自然科学基金委员会等资助；感谢厦门大学在佐治亚技术学院期间对研究项目的财政支援。