  We analyze robustness of decoherence-free (DF) subspace in charge qubits when
there are a local structure and non-uniformity that violate collective
decoherence measurement condition. We solve master equations of up to four
charge qubits and a detector as two serially coupled quantum point contacts
(QPC) with an island structure. We show that robustness of DF states is
strongly affected by local structure as well as by non-uniformities of qubits.
