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EPJ Quantum Technology

Decoy state semi-quantum key distribution

Shuang Dong, Shang Mi, Qing-Cheng Hou, Yutao Huang, Jindong Wang, Ya‐Fei Yu, Zhengjun Wei, Zhiming Zhang, Junbin Fang

Published May 31, 2023• 17 citations
Publisher

Abstract

Abstract Semi-quantum key distribution describes a system in which a fully quantum user and classical user perform key distribution. The main advantage of key distribution is its security. Owing to the bottlenecks of existing technology, highly attenuated lasers and threshold detectors are required for semi-quantum key distribution; however, these components make semi-quantum key distribution susceptible to eavesdroppers. Our previous study presented the first semi-quantum key distribution experiment and verified the feasibility of the mirror protocol in 2021. Herein, we first build a semi-quantum key distribution channel model and use Gottesman-Lo-Lütkenhaus-Preskill theory to evaluate its safety performance in the case of a quasi-single photon source. Moreover, we determine that an eavesdropper can steal all information through the photon-number-splitting attack without being detected. Therefore, we add decoy states to the semi-quantum key distribution to estimate the furthest transmission distance and secure bit rate under asymptotic conditions. Semi-quantum key distribution can still be achieved safely with highly attenuated lasers and threshold detectors in 150 km.

DOI
10.1140/epjqt/s40507-023-00175-0

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Abstract Semi-quantum key distribution describes a system in which a fully quantum user and classical user perform key distribution. The main advantage of key distribution is its security. Owing to the bottlenecks of existing technology, highly attenuated lasers and threshold detectors are required for semi-quantum key distribution; however, these components make semi-quantum key distribution susceptible to eavesdroppers. Our previous study presented the first semi-quantum key distribution experiment and verified the feasibility of the mirror protocol in 2021. Herein, we first build a semi-quantum key distribution channel model and use Gottesman-Lo-Lütkenhaus-Preskill theory to evaluate its safety performance in the case of a quasi-single photon source. Moreover, we determine that an eavesdropper can steal all information through the photon-number-splitting attack without being detected. Therefore, we add decoy states to the semi-quantum key distribution to estimate the furthest transmission distance and secure bit rate under asymptotic conditions. Semi-quantum key distribution can still be achieved safely with highly attenuated lasers and threshold detectors in 150 km.
Semi-quantum key distribution is based on the basic principle of quantum mechanics, which allows a classical user and quantum user to use information theory to have a secure shared key. In 2021, our research group proved the first proof-of-principle experimental demonstration of semi-quantum key distribution and verified its feasibility. Due to the limitations of existing science and technology, the experimental system still features a combination of multiphoton signal source and loss in the transmission line. This makes semi-quantum key distribution as susceptible to a photon-number splitting attack as quantum key distribution, leading to limitations of secure transmission distance. It seems that practical single-state semi-quantum key distribution can overcome photon-number splitting attack due to the SIRT bits (also known as the “sifted key”). However, its dual-channel feature still opens up an observation window to Eve. We present two joint photon-number splitting attacks suitable for a single-state semi-quantum key distribution system and show that through the joint photon-number splitting attack, Eve can obtain key information without being detected by Alice or Bob.
Quantum key distribution establishes a secret string of bits between two distant parties. Of concern in weak laser pulse schemes is the especially strong photon number splitting attack by an eavesdropper, but the decoy state method can detect this attack with current technology, yielding a high rate of secret bits. In this Letter, we develop rigorous security statements in the case of finite statistics with only a few decoy states, and we present the results of simulations of an experimental setup of a decoy state protocol that can be simply realized with current technology.
The passive decoy scheme represents a quantum key distribution protocol that employs a photon number resolving detector to selectively extract the decoy states from the signal states during post-processing. This selection process ensures resilience against attacks capable of distinguishing between these two states, thereby enhancing the security of the protocol. Previous research has emphasized the good performance of the protocol under optimal conditions. However, our study focuses on numerically evaluating the protocol performance in various practical scenarios. We aim to understand the implications of these scenarios in terms of the maximum attainable attenuation and bit rate per second, considering the number of photons per pulse sent under specific experimental conditions. Our findings demonstrate that opting for a low average photon number value ensures high tolerance to attenuation during communication, albeit at the cost of a reduced bit rate per second. Conversely, for lower attenuations, it is more advantageous to increase the average number of photons, even if it deviates from the ideal behaviour of a single-photon source. The benefit of selecting the decoy states at the end of the transmission remains consistent across different configurations. This research signifies an advancement in assessing the viability of incorporating the protocol within a practical QKD network.
The theoretical existence of photon-number-splitting attacks creates a security loophole for most quantum key distribution (QKD) demonstrations that use a highly attenuated laser source. Using ultralow-noise, high-efficiency transition-edge sensor photodetectors, we have implemented the first version of a decoy-state protocol that incorporates finite statistics without the use of Gaussian approximations in a one-way QKD system, enabling the creation of secure keys immune to photon-number-splitting attacks and highly resistant to Trojan horse attacks over 107 km of optical fiber.
Problem statement: Quantum key distribution provides unconditional security guaranteed by the fundamental laws of quantum physics. Unfortunately, for real-life experimental set-ups, which mainly based on faint laser pulses, the occasional production of multi-photons and channel loss make it possible for sophisticated eavesdroppers to launch various subtle eavesdropping attacks including the Photon Number Splitting (PNS) attack. The decoy state protocols recently proposed to beat PNS attack and to improve dramatically distance and secure key generation rate of Quantum Key Distribution (QKD). Approach: Objective of this study was experimental implementation of weak decoy + vacuum states QKD for increasing the performance of QKD system. To show conceptually how simple it was to apply the weak decoy + vacuum state idea to a commercial QKD system, we chosen ID-3000 commercial quantum key distribution system manufactured by id quantique. To implement the weak decoy + vacuum state protocol, we had to add some new optical and electronics components to id quantique and to attenuate each signal to the intensity of either signal state or weak decoy or vacuum state randomly. Results: In our implementation, the attenuation will be done by placing a VOA (variable optical attenuator) in Alice’s side. Specifically, our QKD system required the polarizations of 2 pulses from the same signal to be orthogonal. Therefore the VOA must be polarization independent so as to attenuate the two pulses equally. The VOA utilized in experiment to attenuate signals dynamically was Intensity Modulator (IM). We had implemented weak + vacuum protocol on a modified commercial QKD system over a 25 km of telecom fibers with an unconditionally secure key rate of 6.2931×10-4 per pulse. Conclusion: By making simple modifications to a commercial quantum key distribution system, we could achieve much better performance with substantially higher key generation rate and longer distance than QKD system without decoy state.
To increase dramatically the distance and the secure key generation rate of quantum key distribution (QKD), the idea of quantum decoys--signals of different intensities--has recently been proposed. Here, we present the first experimental implementation of decoy state QKD. By making simple modifications to a commercial quantum key distribution system, we show that a secure key generation rate of 165 bit/s, which is 1/4 of the theoretical limit, can be obtained over 15 km of a telecommunication fiber. We also show that with the same experimental parameters, not even a single bit of secure key can be extracted with a non-decoy-state protocol. Compared to building single photon sources, decoy state QKD is a much simpler method for increasing the distance and key generation rate of unconditionally secure QKD.