Quantum-information storage: A Schrodinger-picture approach

A theory is developed to obtain the state vector for an atom-field system in which a single-photon pulse propagates in an optically dense ensemble of two-level or three-level atoms. The manner in which the field and the atoms become entangled is described, as is the entanglement of the atoms produced by the radiation pulse. Two protocols for quantum-information storage are studied. In the first protocol, the single-photon pulse is totally absorbed in an ensemble of two-level atoms that are embedded in a dielectric host. To arrive at analytical expressions for the state vector, fluctuations of atomic position are neglected and it is assumed that the atoms' transition frequencies are inhomogeneously broadened with a width that is much larger than the bandwidth of the single-photon pulse. In the second protocol, a single-photon pulse is sent into a semi-infinite medium of three-level atoms under the conditions where electromagnetic-induced transparency can occur. It is shown that the usual results of pulse compression and reduced group velocity are recovered. Moreover, an explicit expression for the state vector of the atom-field system is obtained.