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

TL;DR

This paper introduces an optimal universal local quantum cloning machine for two-qubit systems, along with two classes of state-dependent machines, and studies the robustness of quantum discord versus entanglement during broadcasting.

Contribution

It proposes a new optimal universal two-qubit quantum cloning machine and analyzes quantum correlation broadcasting, highlighting discord's robustness over entanglement.

Findings

Quantum discord is more robust than entanglement during broadcasting.

The proposed cloning machine is optimal and universal for two-qubit systems.

State-dependent cloning machines are also characterized.

Abstract

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