Entanglement and Concurrence in the BCS State
  An extension of the notion of concurrence introduced by Wooters is used to quantify the entanglement of certain multipartite pure states, namely, the BCS state of superconducting compounds. This leads to a definition of the macrocanonical entanglement of pairing (MEP) for which we compute an analytical formula in terms of two adimensional numbers, the cut-off and gap numbers, which depend on measurable physical quantities. We find that strongly coupled BCS elements like Pb and Nb have much larger MEP values than more conventional BCS transition metal superconductors. 
M.A. Martin-Delgado