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arXiv:cond-mat/0402393 · Chinese Physics Letters

Thermal Entanglement in Superconducting Quantum-Interference-Device Qubits Coupled to Cavity Field

Pan Hai-Zhong, Kuang Le-Man

Published Feb 19, 2004• 7 citations
arXivHTMLPDF

Abstract

We investigate thermal entanglement in a superconducting-quantum-interference-device qubit coupled to a cavity field. We show that the entanglement can be manipulated by varying temperature and an effective controlling parameter B which depends on the external field and characteristic parameters of the system. We find that there exists a critical value of the controlling parameter B c . Under a fixed temperature, increasing B can increase entanglement in the regime of B < B c , while the entanglement decreases with increasing B in the regime of B > B c .

DOI
10.1088/0256-307x/21/3/002

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Background. Interest in the study of entangled states of systems of natural and artificial atoms (qubits) interacting with selected modes of microwave resonators is associated with their use as logic elements of quantum computers. At the same time, the most important task of the physics of quantum computing is the choice of the most effective mechanisms for manipulating and controlling the entangled states of qubits in such devices. Aim. The dynamics of the entanglement of two dipole-coupled superconducting Josephson qubits induced by a thermal noise of a coplanar resonator is studied for various initial states of the qubits. Methods. Based on the exact solution of the quantum Liouville equation for the whole density matrix of the system under consideration, the time behavior of the qubit entanglement parameter (negativity) is found for chaotic thermal, pure separable, and entangled initial states of qubits. Results. It is shown that the entanglement of qubits induced by the thermal noise of the resonator is possible for both the chaotic thermal states and separable states of qubits, except the case when both qubits are excited. It has also been found that, for small values of the dipole–dipole interaction parameter, taking this interaction into account leads to an increase in the degree of entanglement. For values of the dipole-dipole interaction parameter greater than some limit value, the opposite effect takes place. It is found that for entangled initial states of qubits, the inclusion of direct interaction has a small effect on the entanglement dynamics. It is shown that the initial coherence of qubit states can lead to a significant increase in the degree of their entanglement in the presence of a dipole–dipole interaction. Conclusion. The dipole-dipole interaction can be used as an effective mechanism for qubits entanglement manipulation and controlling.
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