Correction to: Nature Reviews Physics https://doi.org/10.1038/s42254-019-0117-4, published online 28 October 2019.

In the original version of this article, the second affiliation of Albert Roura was missing. The second affiliation, German Aerospace Center (DLR), Cologne, Germany, has now been added. This error has now been corrected in the HTML and PDF versions of the article.