Entanglement quantification made easy: polynomial measures invariant under convex decompositionTools Regula, Bartosz and Adesso, Gerardo (2016) Entanglement quantification made easy: polynomial measures invariant under convex decomposition. Physical Review Letters, 116 . 070504/1-070504/5. ISSN 1079-7114 Full text not available from this repository.AbstractQuantifying entanglement in composite systems is a fundamental challenge, yet exact results are only available in few special cases. This is because hard optimization problems are routinely involved, such as finding the convex decomposition of a mixed state with the minimal average pure-state entanglement, the so-called convex roof. We show that under certain conditions such a problem becomes trivial. Precisely, we prove by a geometric argument that polynomial entanglement measures of degree 2 are independent of the choice of pure-state decomposition of a mixed state, when the latter has only one pure unentangled state in its range. This allows for the analytical evaluation of convex roof extended entanglement measures in classes of rank-two states obeying such condition. We give explicit examples for the square root of the three-tangle in three-qubit states, and show that several representative classes of four-qubit pure states have marginals that enjoy this property.
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