Entanglement Optimization for Pairs of Qubits

Local Operations enhancing the entanglement of bipartite quantum states are of great interest in quantum information processing. Subject of this paper are local selective operations acting on single copies of states. Such operations can lead to larger entanglement with respect to a certain measure as studies before by the Horodeckis and A. Kent et al. (PRA 60,PRL 81 and 83). We present a complete characterisation of all local operations yielding optimal entanglement for pairs of qubits, extending former results of A. Kent et al. We introduce a new technique for the classification of states according to their behaviour under entanglement optimizing operations, using the entanglement properties of the support of density matrices.