Correction to: Nature Physics https://doi.org/10.1038/s41567-018-0277-7, published online 24 September 2018.

In the version of this Article originally published, the angular oscillation of amplitude in Fig. 4a was incorrectly labelled ζ; it should have been ξ. Also, the blue line in the top-right corner of Fig. 6d should not have been dash-dotted but solid. These have now been corrected in all versions of the Article.