Nature Physics 10, 803–813 (2014); published online 31 October 2014; corrected after print 13 November 2014.

In the version of this Review Article originally published, the sources of two images in Fig. 5a were incorrect. The first and second images from the left in Fig. 5a were taken from ref. 114 and ref. 99, respectively. This error has now been corrected in the online versions of the Review Article.