Correction to: Nature https://doi.org/10.1038/s41586-018-0778-7, published online 12 December 2018.

In this Letter, author Christian Gross should have been associated with affiliation ‘1’ (Max-Planck-Institut für Quantenoptik, Garching, Germany) instead of affiliation ‘2’ (Fakultät für Physik, Ludwig-Maximilians-Universität, Munich, Germany); this has been corrected online.