Correction to: npj Quantum Information https://doi.org/10.1038/s41534-018-0084-0.

In the original published version of the Article, the inline equation for the conditional covariance matrix γA|B was listed as γA|B = γAγB,11−1 C Π C. This equation should be γA|B = γA − γB,11−1 σAB Π σAB. This has now been corrected in the PDF and HTML versions of the Article.