Entanglement degree of electron pairs out of Andreev reflection

We study the entanglement degree of electron pairs emitted from an s-wave superconductor, which couples to two normal leads via a single-level quantum dot. Within the framework of scattering matrix theory, the concurrence is used to quantify the entanglement. And the result shows that the entanglement degree is generally influenced by the initial separation of the two electrons in a Cooper pair and the normal transmission eigenvalues T1T1, T2T2. But it is only determined by the eigenvalues in the tunnelling limit, T1,T2≪1T1,T2≪1, what is more, it is measurable.