Björn Piglosiewicz et al. Nature Photonicshttp://dx.doi.org/10.1038/nphoton.2013.288 (2013); published online 19 November 2013; corrected online 19 November 2013.

In the version of this Letter originally published online, one beam was missing in the interferometer section depicted in the schematic shown in Fig. 1a. This error has now been corrected in both the HTML and PDF versions of this Letter.