Decoherence and decoherence suppression in ensemble-based quantum memories for photons

We discuss the effects of decoherence processes on the fidelity of ensemble-based quantum memories for photons and potential ways to suppress them. We analyse individual decoherence processes, where each atom is coupled to an independent reservoir as well as collective reservoir interactions. It is shown that despite the large entanglement of the storage states in the atomic ensemble, the sensitivity to decoherence does in general not increase with the number of atoms. This is due to the existence of equivalence classes of storage states characterised by the same projection onto the relevant quasi-particle modes, which contain the information about the stored photon state. It is shown furthermore that it is possible to construct a collective quantum memory which possesses a two-dimensional decoherence free subspace, provided all atoms are coupled to individual and independent reservoirs. This subspace consists of a pair of special collective states which have a large effective distance in state space. The probability of mutual transitions between these states due to single-atom errors scales like 1/N with N being the total number of atoms. Providing a sufficiently large energy gap to all other states suppresses transitions out of the subspace. In this way qubits can be protected from decoherence due to both, spin flips and dephasing processes.