  We introduce a figure of merit for a quantum memory which measures the
preservation of entanglement between a qubit stored in and retrieved from the
memory and an auxiliary qubit. We consider a general quantum memory system
consisting of a medium of two level absorbers, with the qubit to be stored
encoded in a single photon. We derive an analytic expression for our figure of
merit taking into account Gaussian fluctuations in the Hamiltonian parameters,
which for example model inhomogeneous broadening and storage time dephasing.
Finally we specialize to the case of an atomic quantum memory where
fluctuations arise predominantly from Doppler broadening and motional
dephasing.
