S-matrix approach to the construction of decoherence-free subspaces

Analyzing the S matrices associated with a host of relaxation processes allows one, using extra ‘‘chaperon’’ states, to construct a decoherence-free subspace ~DFS! that is immune to the effects of relaxation. The method does not require knowledge of the system-bath Hamiltonian, which is rarely known. Thus, a DFS can be constructed directly from experimentally determined relaxation and dephasing rates of the relevant decoherence processes. Although the method requires resetting the coefficients of the chaperon states at various times, this resetting does not require determining the coefficients of the states belonging to the DFS that one uses for control, computations, or information storage.