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International Journal of Theoretical Physics

The Relation of Concurrence and Purity for Multi-Qubit of Pure States

Wei Jin, Peilin Zhao, Junling Che, Dan Xiang

Published Jul 10, 2020• 1 citation
Publisher
DOI
10.1007/s10773-020-04535-9

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Knowledge of the relationships among different features of quantumness, like entanglement and state purity, is important from both fundamental and practical viewpoints. Yet, this issue remains little explored in dynamical contexts for open quantum systems. We address this problem by studying the dynamics of entanglement and purity for two-qubit systems using paradigmatic models of radiation-matter interaction, with a qubit being isolated from the environment (spectator configuration). We show the effects of the corresponding local quantum channels on an initial two-qubit pure entangled state in the concurrence–purity diagram and find the conditions which enable dynamical closed formulas of concurrence, used to quantify entanglement, as a function of purity. We finally discuss the usefulness of these relations in assessing entanglement and purity thresholds which allow noisy quantum teleportation. Our results provide new insights about how different properties of composite open quantum systems behave and relate each other during quantum evolutions.
The state of an atom in a bipartite qubit, Jaynes-Cummings (JC) or anti-Jaynes-Cummings (aJC) interaction is described by a reduced density operator. The purity of the state has been measured by taking the trace of the square of the reduced density operator. In this article, we define the square of the reduced density operator as the state purity operator, composed of a completely pure state part and a completely mixed state part. The coefficient of the completely mixed state part is the mixed state measure, formally obtained as the determinant of the reduced density operator and it is therefore directly related to tangle, the square of concurrence of the bipartite system. Expressed in various equivalent forms, the mixed state measure provides all the characteristic elements of state purity or entanglement, such as eigenvalues of the reduced density operator, nonclassicality measures and a state purity complex amplitude. The argument of the state purity complex amplitude in polar form is the phase of the state purity measure, which defines the degree of purity of the state. We find that the degree of purity and concurrence are complementary quantifiers satisfying a complementarity relation.