A characterization of positive linear maps and criteria of entanglement for quantum states

Let H and K be (finite-or infinite-dimensional) complex Hilbert spaces. A characterization of positive completely bounded normal linear maps from B(H) into B(K) is given, which particularly gives a characterization of positive elementary operators including all positive linear maps between matrix algebras. This characterization is then applied to give a representation of quantum channels (operations) between infinite-dimensional systems. A necessary and sufficient criterion of separability is given which shows that a state rho on H circle times K is separable if and only if (Phi circle times I) rho >= 0 for all positive finite-rank elementary operators Phi. Examples of NCP and indecomposable positive linear maps are given and are used to recognize some entangled states that cannot be recognized by the PPT criterion and the realignment criterion.