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Quantum communication with atoms and photons: research towards a quantum repeater


Quantum communication schemes such as dense coding, quantum teleportation and quantum cryptography offer more efficient and more secure ways for the exchange of information in a network. In recent years, quantum communication has received much experimental attention. However, while these schemes are realizable for moderate distances (up to a few tens of kilometres in the case of quantum cryptography), serious problems occur in bringing them to technical useful scales due to the problems caused by photon los ses and decoherence in the transmission channel. Fortunately, as shown in the former references, while the use of entangled photons and of entanglement swapping could offer ways to overcome the photon losses, the decoherence can be overcome by exploiting e ntanglement purification ? a method to distil highly entangled states out of less entangled states. Therefore, quantum repeaters, a combination of entanglement swapping and entanglement purification, hold the promise to solve the problems of the photon lo sses and of the ecoherence in long-distance quantum communication. Most recently, entanglement swapping and entanglement purification have been experimentally demonstrated with high accuracy by using linear optics. However, due to the difficulty to achiev e quantum memory, the implementation of quantum repeaters remains an experimental challenge. Fortunately, a more practical scheme combining linear optics and atomic ensembles has been suggested recently, by which one can, in principle, incorporate entangle ment swapping, entanglement purification and quantum memory together into a united whole. This would thus enable a full realization of quantum repeaters. Exploiting present technologies developed in the recent experiments on multi- photon interferometry an d storage of light in atomic vapour, it will therefore be the main purpose of the present project to investigate the experimental realization of a quantum repeater with atomic ensembles and linear optics. #

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