Nature Photon. 11, 383–389 (2017); published online 29 May 2017; corrected after print 29 June 2017.

In the version of this Article originally published, the following sentences were missing from the Acknowledgements: “Financial support by the Alexander von Humboldt Foundation (Project Tirinto) and the Italian Ministry of Research (project FIRB no. RBID08CRXK) is gratefully acknowledged. A.C. acknowledges financial support from ENS Paris-Saclay.” These have now been added in the online versions of the Article.