  We present a quantum memory protocol that allows to store light in ensembles
of two-level atoms, e.g. rare-earth ions doped into a crystal, by modulating
the refractive index of the host medium of the atoms linearly in time. We show
that under certain conditions the resulting dynamics is equivalent to that
underlying the gradient echo memory protocol, which relies on a spatial
gradient of the atomic resonance frequencies. We discuss the prospects for an
experimental implementation.
