Crosstalk analysis in quantum networks: detection and localization insights with photon counting OTDR

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.