Concurrence (quantum computing)

$mathcal\{C\}(rho)equivmax(0,sqrt\{lambda\_1\}-sqrt\{lambda\_2\}-sqrt\{lambda\_3\}-sqrt\{lambda\_4\})$

in which $lambda\_1,...,lambda\_4$ are the eigenvalues of

$rho(sigma\_\{y\}otimessigma\_\{y\})rho^\{*\}(sigma\_\{y\}otimessigma\_\{y\})$

in decreasing order and $sigma\_y$ is a Pauli spin matrix.

From it one can calculate the entanglement of formation.

References

• Scott Hill and William K. Wootters, Entanglement of a Pair of Quantum Bits, 1997.
• William K. Wootters, Entanglement of Formation of an Arbitrary State of Two Qubits 1998.