摘要： Quantum memories enabling the storage of an input photonic qubit and its later retrieval with a ?delity beating any classical device constitute essential components in quantum communication networks and optical quantum information processing [1]. Our main goal is to develop a new generation of memories which have a near-unity ef?ciency for storage-and-retrieval operations, as well as high multiplexing capabilities. In the recent years, we demonstrated for instance the implementation for quantum bits encoded in the orbital angular momentum degree of freedom, which provides an essential capability for future networks with multimode capability [2]. We also realized multiple-degree-of-freedom memory, which can ?nd applications in classical data processing but also in quantum network scenarios where states structured in phase and polarization have been shown to provide promising attributes [3]. Recently, we realized a multiplexed quantum memory for polarization encoded qubits with high storage-and-retrieval ef?ciency [4]. We report on a quantum memory for polarization qubits that combines an average conditional ?delity above 99% and ef?ciency around 68%, thereby demonstrating a reversible qubit mapping where more information is retrieved than lost. The qubits are encoded with weak coherent states at the single-photon level and the memory is based on electromagnetically-induced transparency in an elongated laser-cooled ensemble of cesium atoms, spatially multiplexed for dual-rail storage. The reported ef?ciency approaches the maximal performance achievable on the D2 line transition used here, as shown by a comprehensive model that includes all the involved atomic transitions. In the alkali-metal atoms, hyper?ne interaction in the excited state indeed introduces several levels which can have a strong effect on the medium susceptibility via off resonant excitation, thereby decreasing the ef?ciency when the optical depth (OD) is large. As shown by this model, switching to the D1 line can increase the ef?ciency above 90%. Preliminary results with optical depth around 400 led us to a storage-and-retrieval ef?ciency of 85% at the single-photon level.