Recently, the research group of Professor Zeng-Bing Chen and Associate Professor Hua-Lei Yin (National Solid State Microstructure Laboratory and School of Physics, Center for Collaborative Innovation of Advanced Microstructures, Nanjing University), cooperating with the Beijing National Laboratory for Condensed Matter Physics and the Institute of Physics of the Chinese Academy of Sciences and other institutes proposed a type-independent quantum key distribution (QKD) protocol four-phase measuring device and implemented a proof-of-principle experiment to prove feasibility. The study ensured the security of the protocol against arbitrary source imperfections and all attacks at the detector level. In addition, the study characterized source imperfections with measurable parameters in experimental implementations. The experimental result showed that the secure key rate could reach 0.25 kbps when the channel loss is 20 dB. Under 10dB channel loss (about 50km optical fiber), the security key rate could reach 91kbps, which can meet one-time encryption requirements for voice calls. Compared to previous device-independent QKD protocols that account for imperfect sources, this research has significantly improved the security key rate and transmission distance, demonstrating the enormous potential for application in the practical deployment of QKD secured with device imperfections. The research result was published in Scientific bulletin with the title of “Measurement-independent experimental-type quantum key distribution with faulty and correlated sources”. [Science Bulletin 67, 2167 (2022)].
Compared to traditional key distribution, QKD allows two remote participants, Alice and Bob, to share secure key bits for encryption and decryption of secret communications. With the one-time pad algorithm, QKD provides theoretical security for information exchanges based on the laws of quantum mechanics. However, for the practical operation of QKD systems, a serious security flaw still exists, which occurs due to the gap between theoretical security assumptions and practical devices. To be precise, a security proof of QKD is established with assumptions about system devices, which cannot be satisfied for realistic devices due to inherent imperfections and eavesdropping disturbance. This discrepancy leads to more information leaking to eavesdroppers, which cannot be noticed by users. To reduce the deviation and further strengthen the security against device faults, a device-independent QKD and a measuring device-independent QKD have been proposed. Device-independent QKD guarantees the unconditional security of QKD by measuring the violation of Bell’s inequality without any device assumptions. Recently, international researchers first performed the device-independent QKD proof-of-principle experiments with studies published in Nature and Physical Review Letters, respectively. However, experimental device-independent QKD implementations still suffer from short transmission distances and are far from implemented in long-distance transmission. Device-independent QKD protocols can successfully bridge all detector gaps by introducing an unreliable middle node for interference measurement. Compared to device-independent QKD protocols, measurement device-independent QKD protocols do not need to make assumptions about the intermediate node with higher secure key rate and longer transmission distance . For example, the current world records of meter-independent QKD protocols are the 404 km two-photon interference meter-independent QKD [Phys. Rev. Lett. 117, 190501 (2016)] made by Hua-Lei Yin et al., and the 833 km single-photon double interference field QKD [Nature Photo. 16, 154 (2022)] produced by Shuang Wang et al. QKD protocols independent of measuring devices are considered the best choice with practical security and efficiency. Therefore, it is important to resolve source imperfections in meter-independent QKD protocols.
There are mainly four types of source imperfections in QKD protocols, including state readiness faults, side channels caused by mode dependency, trojan attacks, and pulse correlations. To solve the shortcomings caused by the above source imperfections, the study adopted the method of the recently proposed reference technique [Sci. Adv. 6, eaaz4487 (2020)] to fully characterize target source imperfections and prove the security of a meter-independent four-phase QKD protocol. Additionally, the study measured parameters characterizing source imperfections and conducted finite key analysis of the protocol to help generate a secure key rate in the experiment.
Additionally, the study implemented a proof-of-principle experiment to prove the feasibility of the protocol. The experiment used the Sagnac loop to automatically stabilize the channel phase fluctuation, and all optical fibers maintain polarization.
The results of the simulation can be seen in the second figure titled “Simulation Key Rate vs. Overall Transmission Loss”. As shown in (a) and (b), with or without side channels, the proposed protocol can still have better performance when the secure key rate is an order of magnitude higher than the previous protocols in the figures. As shown in (c), considering source imperfections, the protocol can generate the secure key rate of 91 kbps with 10 dB channel loss (about 50 km of optical fiber), which can satisfy the requirements voice calls. With a channel loss of 20 dB, the secure key rate can reach 253 bps. When the sources are perfect, the proposed protocol can be demonstrated at over 35 dB, which is a huge improvement in transmission distance in the realm of practical secure QKD. Overall, the study is the first experimental implementation of a QKD protocol using the gold standard method to characterize source imperfections, further proving the feasibility of the method. Considering source imperfections and all attacks upon detection, the four-phase meter-independent QKD protocol can still be used in practical application within the long distance, revealing its potential. to be applied to the future deployment of a practical secure QKD. Future study should solve the disadvantages of Sagnac loop with long distance frequency and phase locked technology and the parameters should be characterized more precisely.
The corresponding authors are Associate Professor Hua-Lei Yin and Professor Zeng-Bing Chen of Nanjing University. This work was supported by the Natural Science Foundation of Jiangsu Province (BK20211145), Basic Research Fund for Central Universities (020414380182), Nanjing Jiangbei New Area Key Research and Development Program (ZDYD20210101 ), the Program for Innovative Talents and Entrepreneurs in Jiangsu (JSSCRC2021484).
See the article:
Experimental type quantum key independent of the measuring device
distribution with faulty and correlated sources
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