Since 1984, various optical quantum key distribution (QKD) protocols have been proposed and examined. In all of them, the rate of secret key generation decays exponentially with distance. A natural and fundamental question is then whether there are yet-to-be discovered optical QKD protocols (without quantum repeaters) that could circumvent this rate-distance tradeoff. This paper provides a major step towards answering this question. We show that the secret-key-agreement capacity of a lossy and noisy optical channel assisted by unlimited two-way public classical communication is limited by an upper bound that is solely a function of the channel loss, regardless of how much optical power the protocol may use. Our result has major implications for understanding the secret-key-agreement capacity of optical channels---a long-standing open problem in optical quantum information theory---and strongly suggests a real need for quantum repeaters to perform QKD at high rates over long distances.
@article{arxiv.1504.06390,
title = {Fundamental rate-loss tradeoff for optical quantum key distribution},
author = {Masahiro Takeoka and Saikat Guha and Mark M. Wilde},
journal= {arXiv preprint arXiv:1504.06390},
year = {2016}
}
Comments
9+4 pages, 3 figures. arXiv admin note: text overlap with arXiv:1310.0129