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  • Open Access

Wide-Spectrum Security of Quantum Key Distribution

Hao Tan1,2,3,*, Mikhail Petrov4,*, Weiyang Zhang5, Liying Han1,2, Sheng-Kai Liao1,2,5, Vadim Makarov2,4,†, Feihu Xu1,2,5,‡, and Jian-Wei Pan1,2,5

  • *These authors contributed equally.
  • †Contact author: makarov@vad1.com
  • ‡Contact author: feihuxu@ustc.edu.cn
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PRX Quantum 6, 040331 – Published 12 November, 2025

DOI: https://doi.org/10.1103/smdh-dcpm

Abstract

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.

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Physics Subject Headings (PhySH)

Corrections

15 May, 2026

Correction: An erroneous funding statement has been removed from the Acknowledgments.

Popular Summary

Quantum key distribution (QKD) enables secure communication by using the principles of quantum mechanics. However, there are gaps between the characteristics of practical devices and their theoretical models. In such cases, an eavesdropper Eve might perform optical attacks and choose her attack wavelengths arbitrarily to maximize the attack performance. This compromises the secure implementation of QKD and limits its large-scale deployment and application. In this study, we propose a wide-spectrum security evaluation methodology to achieve full optical spectrum safety for QKD systems.

We build a testbench that measures the transmittance of fiber-optic components through the 400–2300 nm wavelength range using a supercontinuum bright-light source and spectrometers. We use it to characterize most components in the QKD system. We then consider each attack's light path and calculate its total attenuation. We find the wavelength at which the attack encounters the least resistance and predict the system's security at that wavelength. We also propose an additional physical bandpass filter for the QKD system that strongly suppresses both short and long wavelengths. It restricts the spectral range we need to test.

Our methodology is ready to be used in a certification laboratory. It enables the certification of QKD systems against several important attacks.

Article Text

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Outline

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PRX Quantum 6, 040331– Published 12 November, 2025
  • Received 2 December 2024
  • Revised 18 August 2025
  • Accepted 24 September 2025
  • Corrected 15 May 2026
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