Decoherence and its suppression in collective quantum memories

One possible approach towards a quantum network are photons as information carriers (flying qubits) and atomic ensembles as storage and processing units (stationary qubits). The transfer of quantum states between the photonic and collective atomic Raman excitations is based on an adiabatic rotation of quasi particle excitations, called dark state polaritons. Since the stationary qubits are entangled many particle states, decoherence is expected to be amplified by the number of atoms. However, the collective qubits do not show an enhanced sensitivity to environmental interaction. Furthermore for a successful implementation of a quantum network decoherence effects have to be eliminated. In order to do so decoherence free subspaces can be utilized. This concept has been generalized to the case collective quantum memories and will be discussed in detail.