Quantum error avoiding code based on the decoherence-free subspace
基于无消相干子空间的量子避错码设计

It has been proved that there exist coherence preserving states which are stable under collective decoherence.Such states are called decoherence free states.All the coherence preserving states constitute a subspace named decoherence free subspace(DFS).DFS can be used to construct quantum error avoiding code which is fault tolerant automatically.We first propose a definition for DFS in this paper,and then prove that nearly all definitions are equivalent.We also prove when there may exist DFS.Finally we show a straightforward way to find the base states of DFS utilizing group theory.