Limitations of practical multi-photon decoherence-free states
  It is shown in this paper that decoherence-free subspace (DFS) of practical multi-photon polarization can not avoid the exponential decoherence even in the same extra-environment if the photons are frequency-anticorrelated. The reason lies in that the condition of collective decoherence is not satisfied in this case. As an example, the evolution of biphoton's decoherence-free state is given. Possible solution for feasible multi-photon's DFS state is also given. 
Yong-Sheng Zhang, Chuan-Feng Li, Yun-Feng Huang and Guang-Can Guo