Separability of Mixed Quantum States: Linear Contractions and Permutation Criteria

Recently, a powerful separability criterion was introduced by O. Rudolf in 5] and by K. Chen et al. in 6] – basing on realignment of elements of density matrix. Composing the main idea behind the above criterion and the necessary and sufficient condition in terms of positive maps, we provide a characterization of separable states by means of linear contractions. The latter need not be positive maps. We extend the idea to multipartite systems, and find that, somewhat suprisingly, partial realigment (unlike partial transposition) can detect genuinely tri-parite entanglement. We generalize it by introducing a family of so called permutation separability criteria for multipartite states. Namely, any permutation of indices of density matrix written in product basis leads to a separability criterion. Partial transpose and realignment criterion are special cases of permutation criteria. An early version of the present paper has appeared in e-print archive as quant-ph/0206008. The premutation criterion has been then further developed in 23, 24, 25], where the problem of classification of inequivalent permutation criteria has been investigated.