Concurrence

The concurrence is an entanglement monotone defined for a mixed state of two qubits as

$\mathcal{C}(\rho)\equiv\max(0,\sqrt{\lambda_1}-\sqrt{\lambda_2}-\sqrt{\lambda_3}-\sqrt{\lambda_4})$

in which $λ 1,...,λ 4$ are the eigenvalues of

$\rho(\sigma_{y}\otimes\sigma_{y})\rho^{*}(\sigma_{y}\otimes\sigma_{y})$

in decreasing order and $σ y$ is a Pauli spin matrix.

From it one can calculate the entanglement of formation.

References