This work is supported by the King Abdullah University of Science and Technology; the Singapore Ministry of Education (grants R143000627112 and R143000642112); the Agency for Science, Technology and Research (A*STAR) under contracts 122-PSE-0014 and 1231AFG028 (Singapore); the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP award number NRF-CRP15-2015-03); the National Basic Research Program of China (973 Program, grant number 2015CB932200); the National Natural Science Foundation of China (21635002, 21471109, 21210001 and 21405143); and the Natural Science Foundation of Jiangsu Province (BE2015699). We thank H. Jiang, B. Deng, Z. Fang, Z. Zhou, Y. Zhang, X. Ling, M. Sun and A. Malko for technical assistance.

Reviewer information

Nature thanks R. Comin, W. Heiss and the other anonymous reviewer(s) for their contribution to the peer review of this work.