Correction to: Nature Physics https://doi.org/10.1038/s41567-019-0727-x, published online 23 December 2019.

In the version of this Letter originally published, the following sentence was missing from the Acknowledgements: “D.L., I.I.F., J.G.R. and M.G.T. acknowledge support from UK Quantum Technology Hub for Quantum Communication Technologies funded by EPSRC: EP/M013472/1; programme grant no. EP/L024020/1.” It has now been added in the online versions of this Letter.