  We study the coherent storage and retrieval of a very short multimode light
pulse in an atomic ensemble. We consider a quantum memory process based on the
conversion of a signal pulse into a long-lived spin coherence via light matter
interaction in an on-resonant Lambda-type system. In order to study the writing
and reading processes we analytically solve the partial differential equations
describing the evolution of the field and of the atomic coherence in time as
well as in space. We show how to optimize the process for writing as well as
for reading. If the medium length is fixed, for each length, there is an
optimal value of the pulse duration. We discuss the information capacity of
this memory scheme and we estimate the number of transverse modes that can be
stored as a quantum hologram.
