source instead of an attenuated laser. While such sources are still at a developmental stage QKD has been carried out successfully with them. However, as current sources operate at a low efficiency and frequency key rates and transmission distances are limited. Another solution is to modify the BB84 protocol, as is done for example in the SARG04 protocol, in which the secure key rate scales as formula_13. The most promising solution is the decoy states in which Alice randomly sends some of her laser pulses with a lower average photon number. These decoy states can be used to detect a PNS attack, as Eve has no way to tell which pulses are signal and which decoy. 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.