Nature Photonics 9, 796–808 (2015); published online 27 November 2015; corrected after print 16 December 2016.

In the version of this Review Article originally published, the DOI in the header of the PDF was incorrect; it should have read 'DOI: 10.1038/NPHOTON.2015.201'. This has now been corrected in the online PDF; the HTML version was unaffected.