Concatenating decoherence-free subspaces with quantum error correcting codes

An operator sum representation is derived for a decoherence-free subspace (DFS) and used to (i) show that DFS's are the class of quantum error correcting codes (QECC's) with fixed, unitary recovery operators and (ii) find explicit representations for the Kraus operators of collective decoherence. We demonstrate how this can be used to construct a concatenated DFS-QECC code which protects against collective decoherence perturbed by independent decoherence. The code yields an error threshold which depends only on the perturbing independent decoherence rate. [S0031-9007(99)09301-1].