Entanglement of a Pair of Quantum Bits by Scott Hill and William K. Wootters (1997)

Original 1997 paper introducing the magic-basis concurrence description used here. Primary preprint: arxiv.org/abs/quant-ph/9703041 . Full text: arxiv.org/pdf/quant-ph/9703041 . Eqs.(3)–(6) specify the phased Bell basis and its pure-state entropy relation.