classical control field $\Omega(\tau)$ addresses the ${\ensuremath{|s\rangle}}-{\ensuremath{|1\rangle}}$ transition (see Fig. \[fig:fig1\](a)). The control pulse facilitates the transfer of the quantum field to a spatially-dependent coherence of the two ground states (often called a “spin wave") $S(\tilde{z},\tau)$ as depicted in Fig. \[fig:fig1\](b). The exact physical mechanism that accomplishes this task depends on the scheme. In EIT storage, the control pulse dynamically creates a transparency window in the absorption profile of the ${\ensuremath{|g\rangle}}-{\ensuremath{|1\rangle}}$ transition while adiabatically reducing the group velocity of the quantum field to 0 [@ref:Lukin_RMP_03]. In Raman storage, the control pulse enables the quantum field to be absorbed into ${\ensuremath{|s\rangle}}$ by a two-photon Raman transition [@ref:Nunn_PRA_07]. For photon echo storage, the quantum field promotes an atom to the excited state ${\ensuremath{|1\rangle}}$ and the control pulse transfers the excitation to ${\ensuremath{|s\rangle}}$ by performing a fast $\pi$ pulse. In all cases, the quantum field transfers one atom from ${\ensuremath{|g\rangle}}$ to ${\ensuremath{|s\rangle}}$, however there is no knowledge of which atom. Thus the excitation is coherently distributed over the entire ensemble and it is this collective behavior that makes the process efficient.

The quantum field is now stored in this collective ground state coherence and is therefore sensitive to decoherence processes. These processes, which 