Limitations on quantum key repeaters
Stefan Bäuml, Matthias Christandl, Karol Horodecki, Andreas J. Winter · Nature Communications · 2015
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. Quantum repeaters have been conceived as a means to extend the range of quantum secure communications, but can handle only distillable entangled states. Here, the authors introduce and study quantum key repeaters, devices which may allow long-distance quantum key distribution with non-distillable states.