Optimal broadcasting of mixed states

摘要

The N to M (M >= N) universal quantum broadcasting of mixed states rho(circle times N) is proposed for a qubit system. The broadcasting of mixed states is universal and optimal in the sense that the shrinking factor is independent of the input state and achieves the upper bound. The quantum broadcasting of mixed qubits is a generalization of the universal quantum cloning machine for identical pure input states. A pure state decomposition of the identical mixed qubits rho(circle times N) is obtained.