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Optica Quantum

Noise impact of classical headers on the quantum payload in quantum wrapper networking

Gamze Gül, Gregory S. Kanter, Sze Tiong Tan, Mehmet Berkay On, Roberto Proietti, S. J. Ben Yoo, Prem Kumar

Published May 23, 2025• 2 citations
Publisher

Abstract

The fundamental properties of quantum mechanics, such as the no-cloning theorem, make management and control of quantum networks challenging. Quantum wrapper networking (QWN) offers a solution to this problem by wrapping quantum payloads with classical bits (as headers and/or tails) that can be used to assist with networking, performance monitoring, and probing the properties of the fiber interconnection. Since the classical header and quantum payload travel in the same fiber, one should carefully design the system to mitigate potentially deleterious effects such as noise and cross-talk. In this paper, we identify and characterize the noise induced by the classical headers by in-fiber scattering processes. We study the noise by using a tunable continuous-wave laser to emulate O-band (∼1310 nm) classical headers, which are time- and wavelength-multiplexed with O-band quantum signals. The noise is characterized as a function of the headers’ power, classical-quantum wavelength detuning, and optical fiber length. We demonstrate that the dominant noise contribution originates from spontaneous Raman scattering for wavelength detunings that are larger than 3 nm. We also observe that some noise photons can bleed into the quantum payload due to Rayleigh scattering of backward Raman scattered photons. We further investigate system level impacts of the noise created by classical headers that are produced by employing a small form-factor pluggable transceiver. We demonstrate that the associated noise level has negligible impact on quantum payloads for a 47.8 km deployed fiber link, the longest length used in QWN experiments to date, achieving a coincidences-to-accidentals ratio of 56 and a two-photon interference visibility of 88.8%.

DOI
10.1364/opticaq.558540

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The fundamental properties of quantum mechanics, such as the no-cloning theorem, make management and control of quantum networks challenging. Quantum wrapper networking (QWN) offers a solution to this problem by wrapping quantum payloads with classical bits (as headers and/or tails) that can be used to assist with networking, performance monitoring, and probing the properties of the fiber interconnection. Since the classical header and quantum payload travel in the same fiber, one should carefully design the system to mitigate potentially deleterious effects such as noise and cross-talk. In this paper, we identify and characterize the noise induced by the classical headers by in-fiber scattering processes. We study the noise by using a tunable continuous-wave laser to emulate O-band (∼1310 nm) classical headers, which are time- and wavelength-multiplexed with O-band quantum signals. The noise is characterized as a function of the headers’ power, classical-quantum wavelength detuning, and optical fiber length. We demonstrate that the dominant noise contribution originates from spontaneous Raman scattering for wavelength detunings that are larger than 3 nm. We also observe that some noise photons can bleed into the quantum payload due to Rayleigh scattering of backward Raman scattered photons. We further investigate system level impacts of the noise created by classical headers that are produced by employing a small form-factor pluggable transceiver. We demonstrate that the associated noise level has negligible impact on quantum payloads for a 47.8 km deployed fiber link, the longest length used in QWN experiments to date, achieving a coincidences-to-accidentals ratio of 56 and a two-photon interference visibility of 88.8%.
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.
Quantum wrapper networking (QWN) is an emerging quantum networking protocol that wraps qubits in classical header bits to enable switching/routing, monitoring, and control without detecting the quantum signal. In this work, we encode header bits with two nonorthogonal polarization references to track and actively compensate for the changing birefringence of a 48 km deployed fiber link. Our method is analytical and deterministic, using motorized waveplates and a variable phase retarder to accurately and stably compensate the channel. We verify successful compensation by measuring the polarization stability of single photon qubits and the visibility of entangled photon pairs under both slow birefringence drift due to environmental fluctuations and large sudden changes designed to emulate those that occur during packet switching and rerouting over different fiber paths. For large, sudden changes, our compensator recovers the Stokes vector of single photons to within 10 degrees of the target state on the Poincaré sphere and restores the two-photon interference visibilities to their original levels on the deployed fiber link. Additionally, experiments monitoring long-term compensation over 44 hours show that active polarization compensation maintains the two-photon interference visibility, whereas disabling the compensator leads to visibility degradation over time, particularly in the diagonal-antidiagonal basis, where the average raw visibility decreases by 7.7%. These results add a polarization-control layer to QWN and illustrate that information-carrying headers can enable deterministic physical-layer compensation in the quantum channel over long-distance deployed fiber links.
The coexistence of quantum and classical signals in the same optical fiber is a critical challenge for the deployment of quantum networks. Indeed, selecting an optimal channel for quantum signal transmission is crucial to minimize noise arising from co-propagating classical signals. This work experimentally investigates spontaneous Raman scattering (SpRS), a major source of noise in signals transmitted along the same fiber. Unlike most previous studies relying on narrow-linewidth laboratory lasers or architectures based on spatial or temporal multiplexing of quantum and classical signals, we employ commercial SFP optical transceivers and standard single-core single-mode fiber for the transmission of quantum and classical signals in the same fiber, reflecting conditions typical of deployed urban fiber infrastructures. Building on these measurements, we derive a compact and predictive model that captures the Raman scattering profile, enabling accurate estimation of SpRS noise as a function of source power, wavelength, and fiber length. A key outcome of this work is that the proposed model is independent of the specific optical source used, demonstrating its generality and robustness. The model can therefore be used for the identification of optimal C-band channels for quantum signal allocation, namely those least affected by SpRS noise generated by co-propagating O-band classical traffic. These results pave the way for a parameter-robust description of Raman scattering applicable to diverse fiber-based systems.
The rising interest in quantum-level communication has resulted in proposals for coexistence schemes with classical signals within the same fiber optic channel, where the most recent proposals leverage novel fibers designed for space-division multiplexing (SDM) transmission. In all cases the large power difference between classical and quantum channels presents challenges for such schemes, as the classical signals generate interfering noise that corrupts the quantum signal. In this work, we discuss the main interference mechanisms in coexistence scenarios and provide a model to quantify their impact on the quantum signal quality. Analytical approximations in the model allow accurate and fast numerical solutions in the millisecond time-scale. The model accounts for out-of-band non-linear interference effects, namely spontaneous Raman scattering (SpRS) and four-wave-mixing (FWM) in both cases of single-mode and SDM fibers with weakly-coupled degenerate mode groups. Rayleigh and SpRS backscattering are considered in counter-propagating scenarios. Since broadband classical transmission is targeted, the model also accounts for the effect of stimulated Raman scattering (SRS)-induced power tilt. Use of the model in sample scenarios indicates that the interference noise power is minimized at the high end of the transmission band in both cases were the quantum is co- and counter-propagating with respect to the classical signals, with a preference of one or the other scheme depending on the link length and quantum signal center frequency. Our model reveals that FWM has negligible impact in counter-propagating schemes, but can be relevant in co-propagating schemes under certain scenarios. Nevertheless, the FWM interference can be mitigated by deallocating the classical signals adjacent to the quantum channel.