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arXiv:2604.16961

Fundamental Limits of Eavesdropper Detection and Localization in Optical Fiber via Stimulated Brillouin Scattering

Kiran Adhikari, Janis Nötzel

quant-phSubmitted Apr 18, 2026• Announced Apr 21, 2026• 0 citations• Revised May 4, 2026 (v2)
v1 · Apr 18, 2026v2 · May 4, 2026
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Abstract

Recent work investigated the use of Stimulated Brillouin Scattering (SBS) to measure changes in fiber parameters, thereby enhancing the security of a Quantum Key Distribution (QKD) system. In this work, we focus solely on the impact of quantum technology on the task of intrusion-detection. We derive an effective input-output model for the SBS interaction, and utilize it to compare three detection methods: First, the established state of the art. Second, a photon-counting based method which will likely be available in the near future and, finally, the ultimate quantum limit. We illustrate the potential benefit from modern quantum technology within two different mathematical frameworks: First by using the quantum error exponent of asymmetric hypothesis testing, and second in the context of parameter-estimation and quantum metrology.

Comments
6 pages, 3 figures

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Recent work investigated the use of Stimulated Brillouin Scattering (SBS) to measure changes in fiber parameters, thereby enhancing the security of a Quantum Key Distribution (QKD) system. In this work, we focus solely on the impact of quantum technology on the task of intrusion-detection. We derive an effective input-output model for the SBS interaction, and utilize it to compare three detection methods: First, the established state of the art. Second, a photon-counting based method which will likely be available in the near future and, finally, the ultimate quantum limit. We illustrate the potential benefit from modern quantum technology within two different mathematical frameworks: First by using the quantum error exponent of asymmetric hypothesis testing, and second in the context of parameter-estimation and quantum metrology.
Stimulated Brillouin scattering (SBS) is a powerful mechanism for distributed sensing in optical fibers, as the Brillouin response is highly sensitive to local material parameters. Assuming the input probe is a quantum state, we develop a quantum-information-theoretic framework for SBS-based fiber sensing and intrusion detection. Starting from a linearized opto-acoustic Hamiltonian, we derive an effective input-output relation for a short fiber segment. We then model each fiber segment as a single-mode bosonic Gaussian channel and the entire fiber as a cascade of quantum channels. We then compute the quantum Fisher information for general SBS-dependent parameters and identify optimal measurement settings to achieve the quantum limit, comparing them with receiver-constrained strategies such as heterodyne detection and photon counting. In addition, we propose an adaptive approach that achieves first-order asymptotic optimality and provides a novel second-order correction to address preliminary miscalibration. As an application, we apply the framework to the fiber-tapping detection problem.
Optical fiber networks are part of the important critical infrastructure and known to be prone to eavesdropping attacks. Hence, cryptographic methods have to be used to protect communication. Quantum key distribution (QKD), at its core, offers information theoretical security based on the laws of physics. In deployments, one has to take into account practical security and resilience. The latter includes the localization of a possible eavesdropper after an anomaly has been detected by the QKD system to avoid denial-of-service. Here, we present an approach to eavesdropper location that can be employed in quantum as well as classical channels using stimulated Brillouin scattering. The tight localization of the acoustic wave inside the fiber channel using correlated pump and probe waves allows discovery of the coordinates of a potential threat within centimeters. We demonstrate that our approach outperforms conventional optical time-domain reflectometry (OTDR) in the task of localizing an evanescent outcoupling of 1% with centimeter precision inside standard optical fibers. The system is furthermore able to clearly distinguish commercially available standard SMF28 from different manufacturers, paving the way for fingerprinted fibers in high-security environments.
While modern communication systems pose a high standard of security using state-of-the-art encryption techniques based on computational hardness problems, quantum key distribution (QKD) offers a unique advantage using the laws of quantum mechanics. A secret key is shared between two parties, using quantum states of light over a quantum channel. Due to the quantum nature of the states, any interception or other way of eavesdropping will manifest in the distortion of the quantum state resulting in a change of accessible parameters. Whenever such a change of parameters is noticed, the shared key is marked unsecure and hashed down largely or, in the worst case, discarded.
Twin-field quantum key distribution (TF-QKD) has recently attracted attention for its ability to overcome the fundamental limits of secret key rate for point-to-point connectivity without quantum repeaters in QKD. Coherent-based TF-QKD or CTF-QKD, which utilizes coherent states for both transmissions and detections, has allowed systems to be designed for existing optical fiber communication networks allowing for improved performance compared to conventional QKD. Here, we report a theoretical study of CTF-QKD security from an eavesdropper. Compared to conventional QKD and TF-QKD systems, CTF-QKD system is not limited to using only single photon sources/detectors allowing this system to have comparable performance and range as current optical fiber networks. Using commercially available simulation software, we verify the efficacy by studying three different preventative measures for different modulation formats to prevent an eavesdropper from obtaining any secret key information. Results show that parameters can be limited to prevent an eavesdropper from obtaining any information. The simulation also demonstrates that the photon number-phase uncertainty principle for coherent states starts to play major role of security at 128-QAM modulation. Additional security measures are also described to detect the presence of an eavesdropper and improve the system integrity.
Classical and quantum fiber-optic communication channels are vulnerable to possible intrusions. Quantum key distribution (QKD) provides a physically fundamental and theoretically secure communication method, but the key rate decays over long distances. Recently proposed by our group, a quantum communication protocol based on the physical estimation of an eavesdropper's influence shows extreme efficiency even at distances of thousands of kilometers. In this paper, we investigate the physical limits of eavesdropper detection using optical time domain reflectometry (OTDR) and demonstrate the successful detection of a 0.01 dB leakage over a distance of 1009 km.
Quantum Key Distribution (QKD) is a potential defense against future quantum attacks that can break traditional encryption algorithms, such as RSA and ECC. The current work is based on the development of the BB84 protocol with the features of utilizing quantum mechanics and eavesdropping detection through signal disturbance. A simulation of the BB84 protocol was built using polarization encoding over a 10 km single-mode optical fiber using OptiSystem software. The system consists of standard telecom equipment, such as a continuous wave laser at a frequency of 193.1 THz, polarization controllers, and single-photon detectors. Both secure and insecure cases were simulated, where an attacker (Eve) had used an intercept-resend attack. The outcome showed significant Bit Error Rate (BER) and Quantum Bit Error Rate (QBER) increases during attacks, thus proving the protocol's vulnerability to interference. In addition, a special intrusion detection system based on BER was used to identify such anomalies in real-time. The simulation illustrates the feasibility of using QKD in today's optical networks to secure communications.