Decoherence-free subspaces: necessity of Dicke limit

Decoherence-free subspaces (DFS) consist of quantum states of a system S that are immune to the effect of the environment E on the system. In practise, because of symmetry restrictions, S is usually composed of more than one particle to realize a DFS. We outline a proof that, under very general assumptions about the system, the environment, and the coupling between S and E, a DFS can only be obtained in the Dicke limit when the particles are located at the same position. The nature of the proof points towards new directions in the search for realizable DFS.