  We investigate the properties of a recently proposed Gradient Echo Memory
(GEM) scheme for information mapping between optical and atomic systems. We
show that GEM can be described by the dynamic formation of polaritons in
k-space. This picture highlights the flexibility and robustness with regards to
the external control of the storage process. Our results also show that, as GEM
is a frequency-encoding memory, it can accurately preserve the shape of signals
that have large time-bandwidth products, even at moderate optical depths. At
higher optical depths, we show that GEM is a high fidelity multi-mode quantum
memory.
