  Long-lived optical quantum memories are of great importance for scalable
distribution of entanglement over remote networks (e.g. quantum repeaters).
Long-lived storage generally relies on storing the optical states as spin
excitations since these often exhibit long coherence times. To extend the
storage time beyond the intrinsic spin dephasing time one can use dynamical
decoupling techniques. However, it has been shown that dynamical decoupling
introduces noise in optical quantum memories based on ensembles of atoms. In
this article a simple model is proposed to calculate the resulting
signal-to-noise ratio, based on intrinsic quantum memory parameters such as the
optical depth of the ensemble. We also characterize several dynamical
decoupling sequences that are efficient in reducing this particular noise. Our
calculations indicate that it should be feasible to reach storage times well
beyond one second under reasonable experimental conditions.
