Recursive Encoding and Decoding of Noiseless Subsystem and Decoherence   Free Subspace
  When the environmental disturbace to a quantum system has a wavelength much larger than the system size, all qubits localized within a small area are under action of the same error operators. Noiseless subsystem and decoherence free subspace are known to correct such collective errors. We construct simple quantum circuits, which implement these collective error correction codes, for a small number $n$ of physical qubits. A single logical qubit is encoded with $n=3$ and $n=4$, while two logical qubits are encoded with $n=5$. The recursive relations among the subspaces employed in noiseless subsystem and decoherence free subspace play essential r\^oles in our implementation. The recursive relations also show that the number of gates required to encode $m$ logical qubits increases linearly in $m$. 
Chi-Kwong Li, Mikio Nakahara, Yiu-Tung Poon, Nung-Sing Sze and   Hiroyuki Tomita
quant-ph