2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/90211We find the necessary and sufficient condition under which two two-qubit mixed states can be purified into a pure maximally entangled state by local operations and classical communication. The optimal protocol for such transformation is obtained. This result leads to a necessary and sufficient condition for the exact purification of $n$ copies of a two-qubit state.Small corrections. Final versionQuantum Physicstext