  We investigate the performance of inhomogeneously broadened spin ensembles as
quantum memories under continuous dynamical decoupling. The role of the
continuous driving field is two-fold: first, it decouples individual spins from
magnetic noise; second and more important, it suppresses and reshapes the
spectral inhomogeneity of spin ensembles. We show that a continuous driving
field, which itself may also be inhomogeneous over the ensemble, can enhance
the decay of the tails of the inhomogeneous broadening distribution
considerably. This fact enables a spin ensemble based quantum memory to exploit
the effect of cavity protection and achieve a much longer storage time. In
particular, for a spin ensemble with a Lorentzian spectral distribution, our
calculations demonstrate that continuous dynamical decoupling has the potential
to improve its storage time by orders of magnitude for the state-of-art
experimental parameters.
