Nature Reviews Physics 2, 13–28 (2020) https://doi.org/10.1038/s42254-019-0118-3 Published online 8 November 2019

This article was originally published with an error in Figure 6. The central schematic “Artificial spin ice” had the right-most spin arrow pointing in the wrong direction. This arrow has now been reversed, to correctly depict a geometrically frustrated spin configuration.