  We analyze the robustness of decoherence-free (DF) subspace and subsystem in
charge qubits, when difference from the collective decoherence measurement
condition is large in the long time period, which is applicable for solid-state
qubits using as a quantum memory. We solve master equations of up to four
charge qubits and a detector as a quantum point contact (QPC). We show that
robustness of DF states is strongly affected by local non-uniformities. We also
discuss the possible two-qubit logical states by exactly solving the master
equations.
