# Decoy State Quantum Key Distribution

26th August 2004

Authors: Hoi-Kwong Lo (University of Toronto), Xiongfeng Ma (University of Toronto), Kai Chen (University of Toronto)

### Abstract

Quantum key distribution (QKD) allows two parties to communicate in absolute security based on the fundamental laws of physics. Up till now, it is widely believed that unconditionally secure QKD with the standard Bennett-Brassard (BB84) protocol is only possible at rather low key generation rate and short distances. Previously proposed methods (including single-photon sources) to extend the distances of BB84 and increase key generation rate are mostly experimental and present daunting experimental challenges. Here, we present a simple theoretical idea that will achieve these goals by using only current technology. Our method is to develop substantially the decoy state idea of Hwang and combine it with standard entanglement distillation approach to security proofs. Our results show that secure QKD is possible at a key generation rate as high as $O (\eta)$ (as opposed to $O(\eta^2)$ in prior art) where $\eta$ is the overall transmission probability of the channel and fiber-based QKD can be made unconditionally secure over 100km.

In summary, we can have the best of both worlds---making the best use of our imperfect experimental apparatus and yet getting the strongest level of security--- unconditional security, which is the Holy Grail of quantum cryptography. Our method is, therefore, a significant step in bridging the big gap between the theory and practice of QKD.