Correction to: Nature Physics https://doi.org/10.1038/s41567-019-0665-7, published online 23 September 2019.

In the version of this Article originally published online, the following sentence was missing from the Acknowledgements: “Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515.” It has now been added.