Protecting entanglement in superconducting qubits

Abstract

When a two-qubit system is initially maximally-entangled, two independent decoherence channels, one per qubit, would greatly reduce the entanglement of the two-qubit system when it reaches its stationary state. We propose a method on how to minimize such a loss of entanglement in open quantum systems. We find that the quantum entanglement of general two-qubit systems with controllable parameters can be protected by tuning both the single-qubit parameters and the two-qubit coupling strengths. Indeed, the maximum fidelity $F_{\max }$ between the stationary entangled state, ${\rho }_{\infty }$, and the maximally-entangled state, ${\rho }_m$, can be about $2/3\approx \max \{\mathrm{tr}({\rho }_{\infty }{\rho }_m)\}=F_{\max }$, corresponding to a maximum stationary concurrence, $C_{\max }$, of about $1/3\approx C({\rho }_{\infty })=C_{\max }$. This is significant because the quantum entanglement of the two-qubit system can be protected, even for a long time. We apply our proposal to several types of two-qubit superconducting circuits, and show how the entanglement of these two-qubit circuits can be optimized by varying experimentally-controllable parameters.

13 pages, 9 figures

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