Mutual information superadditivity and unitarity bounds

TL;DR

This paper explores the superadditivity of mutual information in conformal field theories, revealing how it encodes unitarity bounds and characterizes free fields through entropic properties and modular flow analysis.

Contribution

It derives a general formula for the long-distance mutual information between arbitrary regions, linking it to modular flow and providing explicit results for free fields and specific geometries.

Findings

Mutual information satisfies strong superadditivity from the vacuum's Markov property.

Unitarity bounds are encoded in mutual information inequalities, saturating for free fields.

Explicit formulas for mutual information between spheres and regions on null cones are obtained.

Abstract

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.