Nat. Med.; doi: 10.1038/nm.4059; corrected 24 March 2016

In the version of this article initially published online, there were two errors. There was a typographical error in the text, which should have stated that the 'dark current' is an electrochemical gradient required for photon-induced polarization (rather than depolarization, as incorrectly stated). In addition, some funding sources were inadvertently omitted from the Acknowledgments. The errors have been corrected for the print, PDF and HTML versions of this article.