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      Long-distance quantum communication with atomic ensembles and linear optics

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          Abstract

          Quantum communication holds a promise for absolutely secure transmission of secret messages and faithful transfer of unknown quantum states. Photonic channels appear to be very attractive for physical implementation of quantum communication. However, due to losses and decoherence in the channel, the communication fidelity decreases exponentially with the channel length. We describe a scheme that allows to implement robust quantum communication over long lossy channels. The scheme involves laser manipulation of atomic ensembles, beam splitters, and single-photon detectors with moderate efficiencies, and therefore well fits the status of the current experimental technology. We show that the communication efficiency scale polynomially with the channel length thereby facilitating scalability to very long distances.

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          Most cited references23

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          Observation of coherent optical information storage in an atomic medium using halted light pulses.

          Electromagnetically induced transparency is a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium; a 'coupling' laser is used to create the interference necessary to allow the transmission of resonant pulses from a 'probe' laser. This technique has been used to slow and spatially compress light pulses by seven orders of magnitude, resulting in their complete localization and containment within an atomic cloud. Here we use electromagnetically induced transparency to bring laser pulses to a complete stop in a magnetically trapped, cold cloud of sodium atoms. Within the spatially localized pulse region, the atoms are in a superposition state determined by the amplitudes and phases of the coupling and probe laser fields. Upon sudden turn-off of the coupling laser, the compressed probe pulse is effectively stopped; coherent information initially contained in the laser fields is 'frozen' in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated: the stored coherence is read out and transferred back into the radiation field. We present a theoretical model that reveals that the system is self-adjusting to minimize dissipative loss during the 'read' and 'write' operations. We anticipate applications of this phenomenon for quantum information processing.
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            Experimental quantum teleportation

            Quantum teleportation -- the transmission and reconstruction over arbitrary distances of the state of a quantum system -- is demonstrated experimentally. During teleportation, an initial photon which carries the polarization that is to be transferred and one of a pair of entangled photons are subjected to a measurement such that the second photon of the entangled pair acquires the polarization of the initial photon. This latter photon can be arbitrarily far away from the initial one. Quantum teleportation will be a critical ingredient for quantum computation networks.
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              Purification of Noisy Entanglement and Faithful Teleportation via Noisy Channels

              Two separated observers, by applying local operations to a supply of not-too-impure entangled states ({\em e.g.} singlets shared through a noisy channel), can prepare a smaller number of entangled pairs of arbitrarily high purity ({\em e.g.} near-perfect singlets). These can then be used to faithfully teleport unknown quantum states from one observer to the other, thereby achieving faithful transfrom one observer to the other, thereby achieving faithful transmission of quantum information through a noisy channel. We give upper and lower bounds on the yield \(D(M)\) of pure singlets (\(\ket{\Psi^-}\)) distillable from mixed states \(M\), showing \(D(M)>0\) if \(\bra{\Psi^-}M\ket{\Psi^-}>\half\).
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                Author and article information

                Journal
                2001-05-22
                Article
                10.1038/35106500
                11719796
                quant-ph/0105105
                46b8a147-57b0-4e95-8a40-fdaf8831c172
                History
                Custom metadata
                Nature 414, 413-418 (2001).
                2 tex files (Main text + Supplement), 4 figures
                quant-ph

                Quantum physics & Field theory
                Quantum physics & Field theory

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