  A coherent technique for the control of photon propagation in optically thick
media and its application for quantum memories is discussed. Raman adiabatic
passage with an externally controlled Stokes field can be used to transfer the
quantum state of a light pulse (``flying'' qubit) to a collective
spin-excitation (stationary qubit) and thereby slow down its propagation
velocity to zero. The process is reversible and has a potential fidelity of
unity without the necessity for strongly coupling resonators. A simple
quasi-particle picture (dark-state polariton) of the transfer is presented. The
analytic theory is supplemented with exact numerical solutions. Finally the
influence of decoherence mechanisms on collective storage states, which are
N-particle entangled states, is analyzed.
