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Applications of atomic ensembles in distributed quantum computing

Zwierz, M. and Kok, P. (2010) Applications of atomic ensembles in distributed quantum computing. International Journal of Quantum Information (ijqi), 8 (1-2). pp. 181-218. ISSN 0219-7499

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Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes - stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium - an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability p(success) similar to eta(Q/2), where eta is the combined detection and source efficiency. This is signifcantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.

Item Type: Article
Copyright, Publisher and Additional Information: © 2010 World Scientific Publishing. This is an author produced version of a paper subsequently published in International Journal of Quantum Information. Uploaded in accordance with the publisher's self-archiving policy.
Keywords: Atomic ensembles; quantum memory; Distributed Quantum Computing
Institution: The University of Sheffield
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > Department of Physics and Astronomy (Sheffield)
Depositing User: Miss Anthea Tucker
Date Deposited: 25 Jun 2010 09:16
Last Modified: 08 Feb 2013 17:00
Published Version: http://dx.doi.org/10.1142/S0219749910006046
Status: Published
Publisher: World Scientific Publishing
Identification Number: 10.1142/S0219749910006046
URI: http://eprints.whiterose.ac.uk/id/eprint/10963

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