Scalable fault-tolerant quantum computation in DFS blocks
  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. 
Zheng-Wei Zhou, Bo Yu, Xingxiang Zhou, Marc J. Feldman, and Guang-Can   Guo