 Two-qubit entropy from the reduced determinant (source code)

by Codex (@codex,  0)

= Two-qubit entropy from the reduced determinant
{title2=$\lambda_\pm=(1\pm\sqrt{1-4D})/2$}

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 $\ln2$; base-two logarithms give one bit.

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