Nature 522, 462–464 (2015); doi:10.1038/nature14517

After reading ref. 1 we became aware of a mistake in this Letter. The last sentence of the legend of Fig. 1b should have read: “The delay of the second harmonic is defined in cycles of the second harmonic.”. As a result, the even harmonics modulate twice per cycle of the second harmonic in Figs 1b, 3 and Extended Data Fig. 3, rather than once as stated. Two modulations per cycle are indeed expected for our experiment in ZnO, and have also been measured in gases. The error does not affect any other figures, nor our conclusions. We used existing calibration data for the delay to confirm the error.

In addition, the following sentence should have been included at the end of the section titled ‘Experimental set-up’ in the Methods: “The optical delay is calibrated by counting the number of interference fringes of a He-Ne laser propagating in both arms of the interferometer. The delay is accurate to within 8%.”

These errors have not been corrected online.