The development of quantum networks marks a transformative advance in communication technologies, enabling communication security unattainable through classical signals and the interconnection of quantum computers, sensors, and devices at various scales. Leveraging classical telecommunication infrastructure, such as optical fibers and associated devices, is pivotal for practical quantum network implementation. The Washington Metropolitan Quantum Network Research Consortium (DC-QNet) is developing a quantum network testbed in the Washington, DC area based on this infrastructure. However, adapting classical systems for quantum networks introduces significant challenges, including crosstalk between classical and quantum channels, stringent synchronization precision demands, polarization drift in optical fibers, and the limitations of classical optical devices. We present experimental demonstrations of these challenges and novel solutions, including a new crosstalk characterization technique. Results are derived from real-world DC-QNet infrastructure, showcasing the feasibility and progress of quantum network implementation.
The development of quantum networks marks a transformative advance in communication technologies, enabling communication security unattainable through classical signals and the interconnection of quantum computers, sensors, and devices at various scales. Leveraging classical telecommunication infrastructure, such as optical fibers and associated devices, is pivotal for practical quantum network implementation. The Washington Metropolitan Quantum Network Research Consortium (DC-QNet) is developing a quantum network testbed in the Washington, DC area based on this infrastructure. However, adapting classical systems for quantum networks introduces significant challenges, including crosstalk between classical and quantum channels, stringent synchronization precision demands, polarization drift in optical fibers, and the limitations of classical optical devices. We present experimental demonstrations of these challenges and novel solutions, including a new crosstalk characterization technique. Results are derived from real-world DC-QNet infrastructure, showcasing the feasibility and progress of quantum network implementation.
Quantum networks exhibit enhanced performance compared to their classical counterparts, enabling secure communication and enhanced sensing protocols. Scalable quantum networks must transmit photons over distances ranging from kilometers to metropolitan scales while preserving entanglement. The objective of this thesis is to characterize the sources and switches of real-world deployed quantum networks, which require an intimate knowledge of nonlinear concepts and devices that drive these operating regimes. Importantly, quantum networks are deployed alongside existing classical communication infrastructure to ensure scalability and compatibility with established fiber-based systems. As a result, quantum and classical signals must coexist within the same telecommunication network. Because quantum infrastructure relies on optical fiber, these networks are susceptible to instabilities arising from fiber- induced effects. To fully realize a scalable quantum network, we investigated the quantum physical layer to optimize performance and identify limitations in our networking system. We characterized the performance of an entangled-photon source used to produce polarization-entangled photons. We then observe how well a high-speed, low-loss quantum switch preserves entanglement, demonstrating its utility for transmitting photons to different nodes in scalable quantum networks. We then analyze a 30 km deployed fiber located in the DC metropolitan area to examine how fiber-induced effects influence network performance. Quantum networks uniquely leverage quantum phenomena to enable fundamentally secure communication and distributed quantum computing beyond the capabilities of classical systems. To realize these advantages, careful characterization of the quantum physical layer and deployed fiber-based networks is essential, as it ensures the stability, reliability, and high-fidelity operation required for large-scale quantum network applications.
The growth of communication networks and the innovation in technology have driven a rising need for advanced security solutions. With the rise of quantum technologies, traditional communication networks could potentially become vulnerable, posing challenges for academia, research, and industry. Quantum Key Distribution (QKD) has emerged as a promising quantum-safe solution, but its integration into existing infrastructures requires extensive study due to challenges in real-world scenarios. This paper presents an innovative quantum-safe infrastructure deployment and an in-depth analysis of co-propagation, using polarization-entangled QKD in the C-band, coexisting with classical data at 100 Gbps via Dense Wavelength Division Multiplexing (DWDM), and employing production-ready equipment within a real infrastructure. Results validate the coexistence of data and quantum channels within the same fibers, demonstrating the adaptability of traditional networks toward future quantum-safe ecosystems. In addition, the paper provides insights and lessons learned for similar deployments.
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.
The coexistence of quantum and classical signals in optical fiber infrastructures represents a major challenge for large-scale quantum networks, as noise sources such as Raman scattering can significantly impact entanglement distribution, and the quantum protocols based on it. In this work, we analyze Raman scattering in the C-band, used for entanglement distribution, generated by a classical the O-band signal. The main contribution of this study lays in investigating these effects in a real metropolitan-fiber network, moving beyond controlled laboratory experiments to deployed telecommunication environments. Measurements are performed over a 7 km metropolitan fiber link using commercial sources and narrowband lasers. The experimental results shows good agreement between the measurements taken under laboratory conditions, although, within the metropolitan-scale loop, localized spectral anomalies are observed in the deployed fibers. Therefore, our results show that, whenever a quantum signal propagates in the C-band alongside an O-band classical channel within the same fiber, careful selection of the operating frequency is required, as Raman scattering and other real-world noise sources can significantly affect the quality and stability of the quantum transmission. In particular, we identify spectral regions that are less affected by Raman noise, thereby providing practical guidelines for optimal quantum channel allocation. We demonstrate that Raman-induced noise constitutes a dominant contribution to the quantum signal-to-noise ratio (SNR) in realistic deployments, beyond background and detector noise. Overall, our findings offer practical insights for deploying quantum communication systems over existing fiber networks, supporting the development of robust and scalable quantum infrastructures.
The need to increase throughput and reduce the latency in the workload distribution and communications between different Data Centers (DCs) demands the use of dedicated or rented fibers. In view of enabling quantum secure communication in centralized and distributed DC, the integration of quantum key distribution (QKD) systems appears as a crucial milestone to safeguard data against quantum computing threats and the "Store-Now, Decrypt-Later" risk. Specifically, tailored technological solutions must be considered to enable practical deployment. In this context, continuous-variable QKD (CV-QKD) presents a promising approach due to its compatibility with optical systems and potential for cost savings and scalability. The adoption of photonic integrated circuits further enhances feasibility by enabling component miniaturization, reducing power consumption, and facilitating large-scale deployment. An important aspect to consider is optimizing fiber usage to support infrastructure sharing and resource efficiency. Therefore, the coexistence of the proposed QKD solutions with inter-DC communications must be carefully assessed. This invited contribution explores the integration of CV-QKD in DC networks to address flexible and efficient quantum secure connectivity. Envisioned co-propagation scenarios and associated challenges will be discussed, offering insights into the evolutionary path towards quantum secure DC infrastructures.