Nature Reviews Physics (2019) https://www.nature.com/articles/s42254-019-0030-x Published online 1 March 2019

This article has been corrected to add a missing image credit to the caption of Fig. 4. The credit line of Fig. 4 now reads “Panel d is adapted from ref.78, CC-BY-4.0 and an image courtesy of Dr Miniaci, Swiss Federal Laboratories for Materials Science and Technology.”