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.
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.
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.
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.
Quantum Key Distribution (QKD) constitutes one of the most profound practical applications of quantum mechanics, enabling two parties to exchange cryptographic keys whose security is guaranteed by the fundamental laws of physics rather than the assumed computational hardness of mathematical problems. As quantum computing capabilities continue to advance, classical public-key infrastructure faces an existential threat, rendering QKD a critical component of post-quantum secure communication architectures. This paper presents a comprehensive survey of QKD spanning foundational protocols, rigorous security proofs, hardware implementation challenges, deployment experiences across metropolitan fiber networks, and recent satellite-based experiments. We analyze the BB84, E91, B92, SARG04, continuous-variable, measurement-device-independent (MDI-QKD), and twin-field (TF-QKD) protocols in depth, examining their security assumptions, achievable key rates, and practical limitations. Particular attention is given to side-channel vulnerabilities in realistic implementations, decoy-state methods for mitigating photon-number-splitting attacks, and finite-key security analysis. We review key performance benchmarks from commercial systems and discuss integration with classical network infrastructure. The paper concludes by identifying open challenges in quantum repeater development, satellite QKD scalability, and standardization efforts essential for large-scale deployment.
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.
Leveraging pre-existing classical fiber optic infrastructure, particularly optical fibers, and their associated component technologies, is essential for the practical realization of future quantum networks. However, when adapting such infrastructure for quantum communication, optical crosstalk from sub-milliwatt classical-channel power into quantum channels presents a significant challenge in quantum network development, introducing substantial noise that limits the network's performance, scalability, and fidelity. Crosstalk can occur between fibers at multiple points, such as connector interfaces at patch panels, within optical devices due to inadequate optical isolation, and in other fiber-related components. Crosstalk also occurs between channels with different frequencies within a single optical fiber, as Raman scattering converts classical light to photons at frequencies that may interfere with the quantum signal. While Raman scattering in optical fibers has been widely studied, crosstalk occurring between separate fibers at their connection locations, typically at network components such as transparent switches and patch panels, in quantum networks has not received sufficient attention. Here we report a demonstration using photon-counting optical time-domain reflectometry (ν-OTDR) to precisely identify and localize crosstalk between separate channels within the same fiber and between separate fibers. The coexistence of classical and quantum signals in the same network necessitates the use of optical switches for efficient routing and control. Crosstalk characterization of an optical switch reveals a strong dependence on the cross-connect configuration. Higher crosstalk levels were observed in certain specific configurations, while no measurable crosstalk was detected in others. Additionally, we found that crosstalk exhibits a pronounced wavelength dependence, increasing over tenfold at longer wavelengths. These findings demonstrate the value of ν-OTDR in diagnosing and mitigating crosstalk in quantum networks. They highlight the importance of optimizing optical switch configurations and wavelength management to minimize noise, ultimately enhancing the scalability, fidelity, and overall performance of quantum networks. This work establishes a foundational approach to addressing crosstalk, paving the way for more robust and efficient quantum network designs.
Increasing incidents of cyber attacks and evolution of quantum computing poses challenges to secure existing information and communication technologies infrastructure. In recent years, quantum key distribution (QKD) is being extensively researched, and is widely accepted as a promising technology to realize secure networks. Optical fiber networks carry a huge amount of information, and are widely deployed around the world in the backbone terrestrial, submarine, metro, and access networks. Thus, instead of using separate dark fibers for quantum communication, integration of QKD with the existing classical optical networks has been proposed as a cost-efficient solution, however, this integration introduces new research challenges. In this paper, we do a comprehensive survey of the state-of-the-art QKD secured optical networks, which is going to shape communication networks in the coming decades. We elucidate the methods and protocols used in QKD secured optical networks, and describe the process of key establishment. Various methods proposed in the literature to address the networking challenges in QKD secured optical networks, specifically, routing, wavelength and time-slot allocation (RWTA), resiliency, trusted repeater node (TRN) placement, QKD for multicast service, and quantum key recycling are described and compared in detail. This survey begins with the introduction to QKD and its advantages over conventional encryption methods. Thereafter, an overview of QKD is given including quantum bits, basic QKD system, QKD schemes and protocol families along with the detailed description of QKD process based on the Bennett and Brassard-84 (BB84) protocol as it is the most widely used QKD protocol in the literature. QKD system are also prone to some specific types of attacks, hence, we describe the types of quantum hacking attacks on the QKD system along with the methods used to prevent them. Subsequently, the process of point-to-point mechanism of QKD over an optical fiber link is described in detail using the BB84 protocol. Different architectures of QKD secured optical networks are described next. Finally, major findings from this comprehensive survey are summarized with highlighting open issues and challenges in QKD secured optical networks.