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arXiv:2512.11701 · Physical Review Applied

Spectral side channels of wavelength-division multiplexer in quantum key distribution under laser damage

Binwu Gao, Junxuan Liu, Ekaterina Borisova, Hao Tan, Mingyang Zhong, Zihao Chen, Qingquan Peng, Weixu Shi, Anastasiya Ponosova, Vadim Makarov, Anqi Huang

quant-phSubmitted Dec 12, 2025• Announced Dec 15, 2025• 0 citations• Revised Sep 8, 2026 (v3)
v1 · Dec 12, 2025v2 · Aug 30, 2026v3 · Sep 8, 2026
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Abstract

In the transmitter of a quantum key distribution (QKD) system, a wavelength-division multiplexer (WDM) is typically used to combine quantum and synchronization signals and is directly connected to the quantum channel. As a result, it becomes the first optical component exposed to laser-injection attacks. Therefore, understanding the behavior of WDMs under such attacks is essential for assessing the practical security of QKD systems. In this work, we systematically investigate the characteristics of WDMs under high-power laser illumination. Our experimental results show that certain WDM samples exhibit pronounced changes in their spectral features once the injected laser power surpasses a specific threshold. Taking the Trojan-horse attack as an illustrative example, we further perform a theoretical analysis of the resulting spectral side channel and show that it can reduce the maximum secure transmission distance to below $66.9\%$ of its original value. By combining experimental observations with theoretical modeling, this study advances the understanding of the influence of WDMs on the practical security of QKD systems.

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In the transmitter of a quantum key distribution (QKD) system, a wavelength-division multiplexer (WDM) is typically used to combine quantum and synchronization signals and is directly connected to the quantum channel. As a result, it becomes the first optical component exposed to laser-injection attacks. Therefore, understanding the behavior of WDMs under such attacks is essential for assessing the practical security of QKD systems. In this work, we systematically investigate the characteristics of WDMs under high-power laser illumination. Our experimental results show that certain WDM samples exhibit pronounced changes in their spectral features once the injected laser power surpasses a specific threshold. Taking the Trojan-horse attack as an illustrative example, we further perform a theoretical analysis of the resulting spectral side channel and show that it can reduce the maximum secure transmission distance to below 66.9%66.9\%66.9% of its original value. By combining experimental observations with theoretical modeling, this study advances the understanding of the influence of WDMs on the practical security of QKD systems.
Wavelength division multiplexing (WDM) of quantum key distribution (QKD) with classical optical channels has been proven to be feasible experimentally. In this paper, we will analyze the practical security of WDM-QKD theoretically. Specifically, we first establish the noise model of classical signals. Then, the influence of classical noises on the performance of decoy-state WDM-QKD is studied with a finite-key security analysis. For numerical simulations, the effect of classical noise-reduction methods for different experimental settings is given. The secret key rate and maximum transmission distance of WDM-QKD is simulated with various sent number of quantum pulses or transmitted power of classical signals. Furthermore, the performance of two kinds of decoy-state WDM-QKD is compared, which shows that the optimal decoy-state protocol for WDM-QKD is related to the sent number of quantum pulses. Since WDM-QKD is desired to reduce the cost of QKD networks and since experiments have already been carried out, our work can not only close the gap between theory and practice, but also be used to optimize the experimental parameters and improve its performance.
Implementations of quantum key distribution (QKD) need vulnerability assessment against loopholes in their optical scheme. Most of the optical attacks involve injecting or receiving extraneous light via the communication channel. An eavesdropper can choose her attack wavelengths arbitrarily within the quantum channel passband to maximize the attack performance, exploiting spectral transparency windows of system components. Here we propose a wide-spectrum security evaluation methodology to achieve full optical spectrum safety for QKD systems. This technique requires transmittance characterization in a wide spectral band with a high sensitivity. We report a testbench that characterizes insertion loss of fiber-optic components in a wide spectral range of 400–2300 nm and up to 70 dB dynamic range. To illustrate practical application of the proposed methodology, we give a full Trojan-horse attack analysis for some typical QKD system configurations and discuss briefly induced-photorefraction and detector-backflash attacks. Our methodology can be used for certification of QKD systems.
Quantum key distribution (QKD) is a promising method for sharing an information-theoretically secure secret key using the properties of quantum mechanics. Although QKD is the most practical field in quantum information science, its widespread adoption is constrained by high implementation costs. Despite the cost-effectiveness of propagating QKD with classical communication using wavelength division multiplexing (WDM), the noise generated by relatively strong classical signals degrades QKD performance. Even though several mitigation strategies exist, they often involve complexity or additional hardware. In this paper, we apply advantage distillation (AD) to enhance the performance of a practical decoy-state BB84 QKD system operating in a WDM environment. We perform finite-key analysis in both co-propagation and counter-propagation scenarios with three classical channels, and furthermore we examine the effects of varying the number of classical channels and the optical power per channel. A comparison of simulation results obtained with and without AD demonstrates that AD is robust to noise originating from classical channels. These results show that AD increases the secure key rate and transmission distance in the WDM-based QKD environment under both co-propagation and counter-propagation without requiring hardware changes. These findings suggest advantage distillation is a promising strategy for the widespread adoption of WDM-based QKD, including in challenging counter-propagation cases.
An eavesdropper Eve may probe a quantum key distribution (QKD) system by sending a bright pulse from the quantum channel into the system and analyzing the back-reflected pulses. Such Trojan-horse attacks can breach the security of the QKD system, if appropriate safeguards are not installed or if they can be fooled by the Eve. We present a risk analysis of such attacks based on extensive spectral measurements, such as transmittance, reflectivity, and detection sensitivity of some critical components used in a typical QKD systems. Our results indicate the existence of wavelength regimes, where the attacker gains considerable advantage as compared to launching an attack at 1550 nm. We also propose countermeasures to reduce the risk of such attacks.
Quantum key distribution (QKD) is a new technique for secure key distribution based on the laws of physics rather than mathematical or algorithmic computational complexity used by current systems. Understanding the compatibility of QKD at 1310 nm with the existing commercial optical networks bearing classical wavelength-division-multiplexed (WDM) channels at 1550 nm is important to advance the deployment of QKD systems in such networks. The minimum wavelength separation for multiplexing QKD and WDM channels on a shared fiber is experimentally determined for impairment-free QKD+WDM transmission.
Quantum key distribution (QKD) enables private communications with information-theoretic security. To guarantee the practical security of QKD, it is essential that QKD systems are implemented in accordance with theoretical requirements and are robust with regard to side-channel attacks. Here we study a prominent attack on QKD transmitters known as the laser-seeding attack (LSA). It consists in injecting photons into the laser of the transmitter in an attempt to modify the outgoing light in some way that is beneficial to the eavesdropper. In this work we measure the response of a QKD transmitter to the LSA as a function of the optical power injected, allowing us to quantify the level of optical attenuation required to mitigate the attack. Further, we use a laser-rate-equation model to numerically simulate the effects of the LSA on a gain-switched laser. With this model we are able to reproduce previous experimental results, as well as generate new insight into the LSA by examining the effects of the LSA when the QKD transmitter is operated with different laser current driving parameters.