Rényi entanglement entropy of the spin-partitioned BCS ground state


We use the Rényi entanglement entropy to study entanglement between the spin components of the Bardeen-Cooper-Schrieffer (BCS) ground state and we characterize its limiting values. We show that Rényi entropies peak in the region about the Fermi surface. We also show that the orbital single-copy entanglement can be interpreted as the thermodynamic free energy of a two-level system at thermal equilibrium. We observed that the total Rényi-2 entropy and the total single-copy entanglement are proportional to the number of electron orbitals having energies about the Fermi energy.