Generating stationary entangled states in superconducting qubits

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 controlled 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(infinity), and the maximally entangled state, rho(m), can be about 2/3 approximate to max{tr(rho(infinity)rho(m))}=F-max, corresponding to a maximum stationary concurrence, C-max, of about 1/3 approximate to C(rho(infinity))=C-max. This is significant because the quantum entanglement of the two-qubit system can be produced and kept, 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.