 Using this idea the secure key rate scales as formula_11, the same as for a single photon source.This idea has been implemented successfully first at the University of Toronto, and in several follow-up QKD experiments, allowing for high key rates secure against all known attacks.Because currently a dedicated fibre optic line (or line of sight in free space) is required between the two points linked by quantum key distribution, a denial of service attack can be mounted by simply cutting or blocking the line.