Master thesis defence by Johannes Borregaard – Niels Bohr Institute - University of Copenhagen

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Master thesis defence by Johannes Borregaard

Long-Distance Entanglement Distribution Using Coherent States

Quantum information theory is an area of physics in which it is studied how quantum mechanics can form a basis for communication and information processing. This has shown impressing results such as teleportation, quantum key distribution and quantum computation.

A key concept in quantum information theory is entanglement, which often has to be established over a large distance in e.g. teleportation and key distribution schemes. Protocols of entanglement distribution are called quantum repeaters and are divided into two groups: Repeaters in the discrete variable regime and repeaters in the continuous variable regime. Recently Jonathan B. Brask et al. have suggested a hybrid repeater protocol that combines the advantages of both the discrete and continuous variable regimes. The performance of this repeater is comparable to the some of the best repeaters in the discrete variable regime but unlike these the hybrid repeater do not rely on highly efficient photodetectors [1].

Nonetheless the hybrid repeater protocol could be improved, which has been the subject of my master thesis. I have studied two ways of improving the original hybrid repeater protocol, which has lead to an altered hybrid repeater protocol. In my defence I will give a brief introduction to the original repeater protocol in Ref. [1] and outline the motivation for the changes leading to the altered repeater protocol.

Furthermore I will present the main elements of the altered repeater and discuss its performance compared to the original protocol. References:[1] J.B. Brask et al: “A Hybrid Long-Distance Entanglement Distribution Protocol”, Phys. Rev. Lett. 105, 160501 (2010)