Marginal independence and an approximation to strong subadditivity
Given a multipartite quantum system, what are the possible ways to impose mutual independence among some of the parties, and the presence of correlations among others, such that there exists a quantum state which satisfies these demands? This question and the related notion of a pattern of marginal independence (PMI) were introduced in arXiv:1912.01041, and then argued in arXiv:2204.00075 to distill the essential information for the derivation of the holographic entropy cone. Here we continue the general analysis initiated in arXiv:1912.01041, focusing in particular on the implications of the necessary condition for the saturation of subadditivity. This condition, which we dub Klein's condition, will be interpreted as an approximation to strong subadditivity for PMIs. We show that for an arbitrary number of parties, the set of PMIs compatible with this condition forms a lattice, and we investigate several of its structural properties. In the discussion we highlight the role played by the meet-irreducible elements in the solution of the quantum marginal independence problem, and by the coatoms in the holographic context. To make the presentation self-contained, we review the key ingredients from lattice theory as needed.
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