The decoy state method in quantum key distribution

Hoi‐Kwong Lo

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 based on standard Bennett-Brassard (BB84) protocol is limited in both key generation rate and distance because of imperfect devices. Here, we solve these two problems directly by presenting new protocols that are feasible with only current technology. Surprisingly, our new protocols can make fiber-based QKD unconditionally secure at distances over 100km (for some experiments, such as GYS) and increase the key generation rate from O(η2) in prior art to O(η) where η is the overall transmittance. Our method is to develop the decoy state idea (first proposed by W.-Y. Hwang in "Quantum Key Distribution with High Loss: Toward Global Secure Communication", Phys. Rev. Lett. 91, 057901 (2003)) and consider simple extensions of the BB84 protocol. This part of work is published in "Decoy State Quantum Key Distribution", . We present a general theory of the decoy state protocol and propose a decoy method based on only one signal state and two decoy states. We perform optimization on the choice of intensities of the signal state and the two decoy states. Our result shows that a decoy state protocol with only two types of decoy states—a vacuum and a weak decoy state—asymptotically approaches the theoretical limit of the most general type of decoy state protocols (with an infinite number of decoy states). We also present a one-decoy-state protocol as a special case of Vacuum+Weak decoy method. Moreover, we provide estimations on the effects of statistical fluctuations and suggest that, even for long distance (larger than 100km) QKD, our two-decoy-state protocol can be implemented with only a few hours of experimental data. In conclusion, decoy state quantum key distribution is highly practical. This part of work is published in "Practical Decoy State for Quantum Key Distribution", . We also have done the first experimental demonstration of decoy state quantum key distribution, over 15km of Telecom fibers. This part of work is published in "Experimental Decoy State Quantum Key Distribution Over 15km", .

Yi Zhao, Hoi‐Kwong Lo, Xiongfeng Ma +3 more

Sudeshna Bhattacharya, Pradeep Kumar Krishnamurthy

We propose a decoy-state method to increase key generation rate of frequency-coded (FC) implementation of B92 quantum key distribution (QKD) protocol. Direct application of decoy-state method to FC-QKD results in a security loophole. We overcome this loophole by optimizing the ratio of decoy-to-signal carrier photon number to unity. Simulation results for a single-channel FC-QKD show 16 kbits of raw key length at a transmission distance of 70 km. We then extend the decoy-state method to subcarrier-multiplexed FC-QKD. We show that key generation rate and secure link length improves by a factor of 5 and 40 km, respectively, compared to single-carrier with decoy-state method.

Jesni Shamsul Shaari, Iskandar Bahari, Sellami Ali

Fen-Zhuo Guo, Li Liu, An-Kang Wang +1 more

Xuejun Zhou

Based on the model of quantum key distribution(QKD) system with a heralded single photon source(HSPS),three typical BB84-decoy-state methods are introduced including active decoy-state protocol, passive decoy-state protocol and the protocol combining active and passive decoy-state idea.The performance and implementation difficulty of the three decoy-state methods are analyzed with numerical simulation.The results show that the key generation rate of BB84 decoy-state protocol is increased by the passive decoy idea.Passive decoy-state method proposed by Adachi,Yamamoto,Koashi and Imoto(AYKI) is easy to implement and speedy to distribute keys because there is no need to prepare decoy states actively. Furthermore,the performance of the AYKI protocol is close to that of the infinite decoy-state protocol. The performance of the method combining active and passive decoy-state idea is better than that of AYKI protocol,but there is not much room left for improvement after the AYKI protocol is implemented.