Nature Communications 3: Article number: 641 (2012); Published: 24 January 2012; Updated: 5 February 2013.

The funding information for this Article was not fully acknowledged. The Acknowledgements should have read:

We are grateful to F. Werner and D. Cohen for enlightening discussions on the subject. Supported by the Office of Naval Research grant N00014-09-1-0502 (M.O. and V.D.) and N00014-09-1-0966 (M.R.), and the National Science Foundation grants PHY-1019197 (M.O. and V.D.) and PHY-0902906 (K.J.).