We thank J. Mei, L. Fu, X. Dai, X. Wang, K. T. Law, D. Yu, F. Zhang, J. Wu and D. L. Deng for valuable discussions. This work was supported by the Guangdong Innovative and Entrepreneurial Research Team Program (2016ZT06D348), NNSFC (11874193), the Shenzhen Fundamental Subject Research Program (JCYJ20170817110751776) and the Innovation Commission of Shenzhen Municipality (KQTD2016022619565991). F.T. thanks the Department of Physics, Renmin University of China for visiting student support. Work at the University of Science and Technology of China, was supported by the National Key R&D Program of China (2016YFA0301700), NNSFC (11474265) and Anhui Initiative in Quantum Information Technologies (AHY170000). Work at Brookhaven is supported by the Office of Basic Energy Sciences, US Department of Energy under contract number DE-SC0012704. The work of K.Y. is supported by National Science Foundation grants DMR-1442366 and DMR-1644779. P.A.L. acknowledges support by the US Department of Energy Basic Energy Sciences grant DE-FG02-03-ER46076. S.A.Y. is supported by the Singapore Ministry of Education AcRF Tier 2 (MOE2015-T2-2-144).

Reviewer information

Nature thanks Alberto Crepaldi, Itamar Kimchi and the other anonymous reviewer(s) for their contribution to the peer review of this work.