Nature Reviews Physics (2020) https://www.nature.com/articles/s42254-020-0157-9 Published online 23 March 2020

In the original version of the article, a typo occurred in the text “unless there exists a shallocdxz 9\aw bound state with energy within this range”. “shallocdxz 9\aw” should read as “shallow”. This has been corrected in the HTML and PDF versions of the manuscript.