Nature 495, 231–235 (2013); doi:10.1038/nature11885

The y axis in Fig. 3h of this Letter was mislabelled owing to a production error. The ticks on the axis should have been labelled ‘0.04’ and ‘0.08’ instead of ‘0.4’ and ‘0.8’, respectively. Figure 3 has been corrected in the HTML and PDF versions of the paper.