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Due to the axioms of quantum mechanics, perfect cloning of an unknown quantumstate is impossible. But since imperfect cloning is still possible, a questionarises: "Is there an optimal quantum cloning machine?" Buzek and Hillery answerto this question and construct their famous B-H quantum cloning machine. TheB-H machine clones state of an arbitrary single qubit in optimal manner andhence it is universal. Generalizing this machine for two-qubit system isstraightforward, but this procedure does not preserve quantum correlationexisting in bipartite state in optimal manner and also, during this procedure,this machine loses its universality and becomes a state-dependent cloningmachine. In this paper we propose an optimal universal local quantum statecloner for two qubit systems. Also we present two classes of state-dependentlocal quantum copying machine. Furthermore, we investigate local broadcastingof two aspects of quantum correlations, i.e., quantum entanglement and quantumdiscord defined, respectively, within the entanglement-separability paradigmand from an information-theoretic perspective. The results show that althoughquantum correlation is in general very fragile during broadcasting procedure,quantum discord is broadcasted more robustly than quantum entanglement.

Two-qubit universal and state-dependent quantum cloning machine and quantum correlation broadcasting