Entanglement, mixedness, and q-entropies

We revisit the relationship between entanglement and purity of states of two-qubits systems, using the q-entropies as measures of the degree of mixture. The q-entropies depend on the density matrix eigenvalues p(i) through the quantity omega(q) = Sigma(i)p(i)(q). Renyi's measures constitutes particular instances of these entropies. We pay particular attention to the case q = 2 and to the limit case q --> infinity. We provide analytical support to numerical results recently reported in the literature. (C) 2002 Elsevier Science B.V. All rights reserved.