Decoherence-free subspaces for a quantum register interacting with a   spin environment
  We study a model of a quantum spin register interacting with an environment of spin particles in quantum-measurement limit. In the limit of collective decoherence we obtain the form of state vectors that constitute high-dimensional decoherence-free subspaces (DFS). In a more general setting we present sufficient and necessary conditions for existence of low-dimensional DFSs that can be used to construct subspaces of higher dimension. 
Pawe{\l} Nale\.zyty, Dariusz Chru\'sci\'nski
quant-ph