  We describe a multi-mode quantum memory for propagating microwave photons
that combines a solid-state spin ensemble resonantly coupled to a frequency
tunable single-mode microwave cavity. We first show that high efficiency
mapping of the quantum state transported by a free photon to the spin ensemble
is possible both for strong and weak coupling between the cavity mode and the
spin ensemble. We also show that even in the weak coupling limit unit
efficiency and faithful retrieval can be obtained through time reversal
inhomogeneous dephasing based on spin echo techniques. This is possible
provided that the cavity containing the spin ensemble and the transmission line
are impedance matched. We finally discuss the prospects for an experimental
implementation using a rare-earth doped crystal coupled to a superconducting
resonator.
