Correction to: Nature Photonics, published online 15 March 2021.

In the version of this Article originally published, in the penultimate sentence of the abstract the units of peak brightness were incorrectly given as photons s–1 mm–1 mrad–1 0.1%BW–1; they should have read photons s–1 mm–2 mrad–2 0.1%BW–1. This has now been corrected in the online versions of the Article.