  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.
