“Stopping” of Light and Quantum Memories for Photons
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.