Correction to: Nature Photonics https://doi.org/10.1038/s41566-018-0281-6, published online 29 October 2018.

In the version of this Letter originally published online, the ORCID number, 0000-0002-8900-3237, of the author A. Josten was missing; and in Fig. 2b, in the y axis label, ‘×105’should have been ‘×103’. These errors have now been corrected in all versions.