  In this thesis, after a brief review of some concepts of Quantum Optics, we
analyze a three-level atomic system in the conditions of electromagnetically
induced transparency (EIT), and we investigate the propagation of a gaussian
pulse along a cigar-shaped cloud of both cold and hot atoms in EIT regime. In
particular, we show that it is possible to amplify a slow propagating pulse
without population inversion. We also analyze the regime of anomalous light
propagation showing that it is possible to observe superluminal energy
propagation. In these conditions, it is possible to imprint reversibly
('write') the information carried by the photons onto the atoms, specifically
as a coherent pattern of atomic spins, and later the information stored in the
atomic spins can be transferred back ('read') to the light field, implementing
in this way a quantum memory. Besides, we analyze the propagation of a quantum
field in an EIT medium sustaining dark state polaritons (DSP) in a
quasi-particle picture. Here, the decoherence effects in this quantum memory
for photons, by analyzing the fidelity of the quantum state transfer, and the
emergence of parastatistics in the quasi-particle picture in gain medium are
discussed. Finally, we introduce a polarization quantum memory for photons by
using a tripod atomic configuration in which two ideal EIT windows appear and
the two DSPs, scattering each other, show a solitonic behavior.
