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Secrecy content of two-qubit states

A. Acín·J. Bae·E. Bagan·M. Baig·Ll. Masanes·R. Muñoz-Tapia
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Abstract

We analyze the set of two-qubit states from which a secret key can be extracted by single-copy measurements plus classical processing of the outcomes. We introduce a key distillation protocol and give the corresponding necessary and sufficient condition for positive key extraction. Our results imply that the critical error rate derived by Chau, Phys. Rev. A {\bf 66}, 060302 (2002), for a secure key distribution using the six-state scheme is tight. Remarkably, an optimal eavesdropping attack against this protocol does not require any coherent quantum operation.

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A. Acín, J. Bae, E. Bagan, M. Baig, Ll. Masanes, R. Muñoz-Tapia. 2004-11-12. Secrecy content of two-qubit states. https://doi.org/10.1103/physreva.73.012327

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