Mutual information superadditivity and unitarity bounds

We derive the property of strong superadditivity of mutual information arising from the Markov property of the vacuum state in a conformal field theory and strong subadditivity of entanglement entropy. We show this inequality encodes unitarity bounds for different types of fields. These unitarity bounds are precisely the ones that saturate for free fields. This has a natural explanation in terms of the possibility of localizing algebras on null surfaces. A particular continuity property of mutual information characterizes free fields from the entropic point of view. We derive a general formula for the leading long distance term of the mutual information for regions of arbitrary shape which involves the modular flow of these regions. We obtain the general form of this leading term for two spheres with arbitrary orientations in spacetime, and for primary fields of any tensor representation. For free fields we further obtain the explicit form of the leading term for arbitrary regions with boundaries on null cones.

Automated summary

The strong superadditivity property of mutual information is derived from the Markov property of the vacuum state in a conformal field theory and strong subadditivity of entanglement entropy. This inequality encodes unitarity bounds for different types of fields, which saturate for free fields. The continuity property of mutual information characterizes free fields, and a general formula for the leading long-distance term is obtained for regions of arbitrary shape involving the modular flow of these regions.