Unique decompositions, faces, and automorphisms of separable states

TL;DR

This paper characterizes the structure and automorphisms of the set of separable quantum states, identifying unique decompositions, faces, and symmetries, which enhances understanding of quantum entanglement and state classification.

Contribution

It provides a detailed description of the faces and automorphisms of the set of separable states, including conditions for unique decompositions and the structure of automorphisms preserving entanglement.

Findings

Dense subset of states with unique convex decompositions identified

Faces of the separable state space characterized as simplexes or direct sums

Automorphisms preserving entanglement and separability explicitly described

Abstract

Let S_k be the set of separable states on B(C^m \otimes C^n) admitting a representation as a convex combination of k pure product states, or fewer. If m>1, n> 1, and k \le max(m,n), we show that S_k admits a subset V_k such that V_k is dense and open in S_k, and such that each state in V_k has a unique decomposition as a convex combination of pure product states, and we describe all possible convex decompositions for a set of separable states that properly contains V_k. In both cases we describe the associated faces of the space of separable states, which in the first case are simplexes, and in the second case are direct convex sums of faces that are isomorphic to state spaces of full matrix algebras. As an application of these results, we characterize all affine automorphisms of the convex set of separable states, and all automorphisms of the state space of B(C^m otimes C^n). that preserve entanglement and separability.