title: Limitations on quantum key repeaters.
abstract: A major application of quantum communication is the distribution of entangled particles for use in quantum key distribution. Owing to noise in the communication line, quantum key distribution is, in practice, limited to a distance of a few hundred kilometres, and can only be extended to longer distances by use of a quantum repeater, a device that performs entanglement distillation and quantum teleportation. The existence of noisy entangled states that are undistillable but nevertheless useful for quantum key distribution raises the question of the feasibility of a quantum key repeater, which would work beyond the limits of entanglement distillation, hence possibly tolerating higher noise levels than existing protocols. Here we exhibit fundamental limits on such a device in the form of bounds on the rate at which it may extract secure key. As a consequence, we give examples of states suitable for quantum key distribution but unsuitable for the most general quantum key repeater protocol.
authors.affiliation: 1] Department of Mathematics, University of Bristol, Bristol BS8 1TW, UK [2] Fisica Teòrica: Informació i Fenòmens Quàntics, Universitat Autònoma de Barcelona, ES-08193 Bellaterra (Barcelona), Spain.
authors.forename: Stefan
authors.initials: S
authors.lastname: Bäuml
authors.affiliation: Department of Mathematical Sciences, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark.
authors.forename: Matthias
authors.initials: M
authors.lastname: Christandl
authors.affiliation: 1] Institute of Informatics, University of Gdańsk, 80-952 Gdańsk, Poland [2] National Quantum Information Centre of Gdańsk, 81-824 Sopot, Poland.
authors.forename: Karol
authors.initials: K
authors.lastname: Horodecki
authors.affiliation: 1] Department of Mathematics, University of Bristol, Bristol BS8 1TW, UK [2] Fisica Teòrica: Informació i Fenòmens Quàntics, Universitat Autònoma de Barcelona, ES-08193 Bellaterra (Barcelona), Spain [3] ICREA-Institució Catalana de Recerca i Estudis Avançats, ES-08010 Barcelona, Spain.
authors.forename: Andreas
authors.initials: A
authors.lastname: Winter