Correction to: Nature Photonics https://doi.org/10.1038/s41566-019-0509-0, published online 2 September 2019.

In the version of this Article originally published, in the Acknowledgments, grant ‘11690032’ from the National Natural Science Foundation of China was mistakenly not mentioned; this information has now been added to the sentence beginning “This research is supported by the...”. The online versions of the Article have been amended.