  Inhomogeneously broadened spin ensembles play an important role in
present-day implementation of hybrid quantum processing architectures. When
coupled to a resonator such an ensemble may serve as a multi-mode quantum
memory for the resonator field, and by employing spin-refocusing techniques the
quantum memory time can be extended to the coherence time of individual spins
in the ensemble. In the present paper we investigate such a memory protocol
capable of storing an unknown resonator-field state, and we examine separately
the various constituents of the protocol: the storage and read-out part, the
memory hold time with the spin ensemble and resonator field decoupled, and the
parts employing spin refocusing techniques. Using both analytical and numerical
methods we derive how the obtainable memory performance scales with various
physical parameters.
