Correction to: Nature https://doi.org/10.1038/s41586-018-0057-7, published online 02 May 2018.

In this Letter, Alexander Groisman should have been listed as an author, with the affiliation Department of Physics, University of California, San Diego, La Jolla, CA, USA. This has been corrected online and his contribution (design of the microfluidic cells) has been moved from the Acknowledgements section to the Author Information section. (The year of ref. 15 has also been corrected.)