  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 $F_{\rm max}$
between the stationary entangled state, $\rho_{\infty}$, and the
maximally-entangled state, $\rho_m$, can be about $2/3\approx\max\{{\rm
tr}(\rho_{\infty}\rho_m)\}=F_{\rm max}$, corresponding to a maximum stationary
concurrence, $C_{\rm max}$, of about $1/3\approx C(\rho_{\infty})=C_{\rm max}$.
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.
