  We investigate how to concatenate different decoherence-free subspaces (DFSs)
to realize scalable universal fault-tolerant quantum computation. Based on
tunable $XXZ$ interactions, we present an architecture for scalable quantum
computers which can fault-tolerantly perform universal quantum computation by
manipulating only single type of parameter. By using the concept of
interaction-free subspaces we eliminate the need to tune the couplings between
logical qubits, which further reduces the technical difficulties for
implementing quantum computation.
