Checking $2\times M$ separability via semidefinite programming

Abstract

In this paper we propose a sequence of tests which gives a definitive test for checking $2\times M$ separability. The test is definitive in the sense that each test corresponds to checking membership in a cone, and that the closure of the union of all these cones consists exactly of {\it all} $2\times M$ separable states. Membership in each single cone may be checked via semidefinite programming, and is thus a tractable problem. This sequential test comes about by considering the dual problem, the characterization of all positive maps acting ${\mathbb{C}}^{2\times 2}\to {\mathbb{C}}^{M\times M}$. The latter in turn is solved by characterizing all positive quadratic matrix polynomials in a complex variable.

4 pages. Phys. Rev. A., to appear

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