Nature Photon. 7, 142–146 (2013); published online 20 January 2013; corrected after print 24 January 2013.

In the version of this Article originally published, the author affiliations were incorrect and should have read: 1IBM TJ Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, USA, 2Institute for Quantum Computing and Department of Applied Mathematics, University of Waterloo, ON, Canada, N2L3G1.

This has now been corrected in the HTML and PDF versions of the Article.