title: Fundamental rate-loss tradeoff for optical quantum key distribution.
abstract: 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. Here 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.
authors.affiliation: 1] National Institute of Information and Communications Technology, Koganei, Tokyo 184-8795, Japan [2] Quantum Information Processing Group, Raytheon BBN Technologies, Cambridge, Massachusetts 02138, USA.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Takeoka
authors.affiliation: Quantum Information Processing Group, Raytheon BBN Technologies, Cambridge, Massachusetts 02138, USA.
authors.forename: Saikat
authors.initials: S
authors.lastname: Guha
authors.affiliation: Department of Physics and Astronomy, Center for Computation and Technology, Hearne Institute for Theoretical Physics, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
authors.forename: Mark M
authors.initials: MM
authors.lastname: Wilde