Two-qubit entropy from the reduced determinant (${\lambda }_{\pm }=(1\pm \sqrt{1-4D})/2$)

For a unit-trace two-dimensional reduced state of a bipartite pure state, determinant $D$ gives eigenvalues $(1\pm \sqrt{1-4D})/2$. Their binary entropy quantifies entanglement. Natural logarithms give a maximum $\ln 2$; base-two logarithms give one bit.