Correction to: Nature Physics 12, 901–906 https://doi.org/10.1038/nphys3865 (2016), published online 22 August 2016.
In the version of this Progress Article originally published, the left and right panels of Fig. 3, clarifying the details indicated within the centre panel, were mistakenly interchanged. This has now been corrected in all versions of the Progress Article.
The original article can be found online at https://doi.org/10.1038/nphys3865.