Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2602.21671 · Journal of Lightwave Technology

Secret Key Rate Limits in Coexisting Classical-Quantum Optical Links

Lucas Alves Zischler, Amirhossein Ghazisaeidi, Antonio Mecozzi, Cristian Antonelli

quant-phSubmitted Feb 25, 2026• Announced Feb 26, 2026• 0 citations
arXivHTMLPDF

Abstract

Classical-quantum coexistence enables cost-effective transmission of data and quantum signals over the same fiber-optic channel. Nevertheless, weak quantum-key distribution (QKD) signals are susceptible to non-linear interference generated from the classical traffic, primarily spontaneous Raman scattering (SpRS) and four-wave-mixing (FWM), as well as to unfiltered noise. In QKD protocols, increased channel loss and excess noise both reduce the secret key rates (SKRs), as illustrated in this work for the two-state BB84 and Gaussian-modulated coherent-states (GMCS) protocols. In this study, we derive closed-form expressions for evaluating the accumulated interference power from coexisting classical signals in a quantum frequency channel. Our model enables effective design of classical-quantum systems in single-mode fibers (SMFs), capturing the evolution of interference arising from the relevant physical phenomena. We utilize the model to examine frequency allocation in multiband transmission systems, demonstrating that, contrary to common practice of allocating QKD channels in the O-band, increased SKR is achieved by placing quantum channels in the upper E-/lower S-band across the relevant scenarios.

Journal reference
Journal of Lightwave Technology, 2026
DOI
10.1109/JLT.2026.3723008
Comments
Submitted for the Journal of Lightwave and Technology

Related papers

Accurate and Effective Model for Coexistence of Classical and Quantum Signals in Optical Fibers

Lucas Zischler, Çağla Özkan, Tristan Vosshenrich +10 more

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.

arXiv:2510.02807•Journal of Lightwave Technology•Aug 4, 2026•0 citations

Time-Interleaved C-Band Co-Propagation of Quantum and Classical Channels

Jing Wang, Brian J. Rollick, Zhensheng Jia +1 more

A successful commercial deployment of quantum key distribution (QKD) technologies requires integrating QKD links into existing fibers and sharing the same fiber networks with classical data traffic. To mitigate the spontaneous Raman scattering (SpRS) noise from classical data channels, several quantum/classical coexistence strategies have been developed. O-band solutions place the QKD channel in the O-band for lower SpRS noise but with the penalty of higher fiber loss and can rarely reach beyond 80 km of fiber; another method is C-band coexistence with attenuated classical channels, which sacrifices the performance of classical channels for lower SpRS noise. In this work, a time-interleaving technique is demonstrated to enable the co- propagation of quantum and classical channels in the C-band without sacrificing either performance. By embedding QKD pulses in the gaps between classical data frames, the quantum channel is isolated from SpRS noise in both wavelength and time domains. C-band co- propagation of a polarization-encoding decoy-state BB84 QKD channel with a 100 Gb/s QPSK channel is experimentally demonstrated with quantum bit error rate (QBER) of 1.12%, 2.04%, and 3.81% and secure key rates (SKR) of 39.5 kb/s, 6.35 kb/s, and 128 b/s over 20, 50, and 100 km fibers, respectively. These results were achieved with the presence of classical launch power up to 10 dBm, which is at least one order of magnitude higher than reported works. We also demonstrated the co- propagation of a QKD channel with eight classical channels with total launch power up to 18-dBm (9-dBm per channel), which is the highest power of classical channels reported in C-band coexistence works.

arXiv:2304.13828•Journal of Lightwave Technology•Mar 26, 2024•33 citations

Optimization of C-band quantum traffic coexisting with O-band classical traffic: preliminary results

Laura d'Avossa, Elena Montella, Marco Grillo +2 more

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.

quant-pharXiv:2607.26905•IEEE International Mediterranean Conference on Communications and Networking (MeditCom)•Jul 30, 2026•0 citations

Enhanced Coexistence of Quantum Key Distribution and Classical Communication over Hollow-Core and Multi-Core Fibers

Weiwen Kong, Yongmei Sun, Tianqi Dou +10 more

In this paper, we investigate the impact of classical optical communications in quantum key distribution (QKD) over hollow-core fiber (HCF), multi-core fiber (MCF) and single-core fiber (SCF) and propose wavelength allocation schemes to enhance QKD performance. Firstly, we theoretically analyze noise interference in QKD over HCF, MCF and SCF, such as spontaneous Raman scattering (SpRS) and four-wave mixing (FWM). To mitigate these noise types and optimize QKD performance, we propose a joint noise suppression wavelength allocation (JSWA) scheme. FWM noise suppression wavelength allocation and Raman noise suppression wavelength allocation are also proposed for comparison. The JSWA scheme indicates a significant enhancement in extending the simultaneous transmission distance of classical signals and QKD, reaching approximately 100 km in HCF and 165 km in MCF under a classical power per channel of 10 dBm. Therefore, MCF offers a longer secure transmission distance compared with HCF when classical signals and QKD coexist in the C-band. However, when classical signals are in the C-band and QKD operates in the O-band, the performance of QKD in HCF surpasses that in MCF. This research establishes technical foundations for the design and deployment of QKD optical networks.

Entropy•Jul 15, 2024•13 citations

Continuous-variable quantum key distribution coexisting with classical signals on few-mode fiber

Hai Zhong, Shanhua Zou, Duan Huang +1 more

Continuous-variable quantum key distribution (CVQKD) holds an advantage of well compatibility with classical coherent optical communications. However, there exists a performance trade-off between CVQKD and classical communication on single-mode fiber (SMF) because of the spontaneous Raman scattering. Space-division multiplexing (SDM) technique may provide a feasible way to mitigate this performance trade-off in short-distance communication while CVQKD coexisting with classical signals on few-mode fiber (FMF). Here, we examine the feasibility of CVQKD coexisting with classical signals on FMF and analyze the noise impact in weak coupling regime. We find that the inter-mode crosstalk generated from the mode coupling and re-coupling between modes and the group delay spread originated from the differential group delay (DGD) contribute the main noise sources. DGD may become one of the main limits for FMF-based CVQKD towards high-speed system. In addition, a well channel wavelength management is needed to suppress the inter-mode four-wave-mixing for achieving the positive secret key rates. The numerical simulations identify the key parameters for CVQKD system, enabling a helpful insight for realizing security analysis of the Gaussian modulated coherent state protocol. It shows that CVQKD coexisting with high power classical signals on FMF is feasible to implement with standard telecommunication components and able to operate at higher secret key rates. The results may provide a potential guideline for the practical high-rate CVQKD integrating with the FMF-based configuration.

Optics Express•Apr 19, 2021•15 citations

Experimental Demonstration of Coexistence of Classical and Quantum Communication in Quantum Key Distribution Link

Gokul Agasthilingam, Harinee Natarajan, Varun Raghunathan

This paper presents a quantum key distribution (QKD) system that combines classical and quantum signals in a single optical channel for quantum key distribution application. We carefully optimize the communication link by placing the bright classical channel wavelength longer than the weak quantum one, number and specification of wavelength division multiplexers, and maximizing on-off extinction in the intensity modulator to minimize crosstalk of bright classical photons onto the quantum channel. Our experiments demonstrate a secret key rate (SKR) of 4.39 kbps for an emulated channel length of 65.5 km with less than 3% quantum bit error rate (QBER). The experimental study discussed here is relevant for utilization of existing optical fiber networks for QKD, making the system more robust for practical usage.

IEEE Microwaves, Antennas, and Propagation Conference (MAPCON)•Dec 11, 2023•2 citations
Classical-quantum coexistence enables cost-effective transmission of data and quantum signals over the same fiber-optic channel. Nevertheless, weak quantum-key distribution (QKD) signals are susceptible to non-linear interference generated from the classical traffic, primarily spontaneous Raman scattering (SpRS) and four-wave-mixing (FWM), as well as to unfiltered noise. In QKD protocols, increased channel loss and excess noise both reduce the secret key rates (SKRs), as illustrated in this work for the two-state BB84 and Gaussian-modulated coherent-states (GMCS) protocols. In this study, we derive closed-form expressions for evaluating the accumulated interference power from coexisting classical signals in a quantum frequency channel. Our model enables effective design of classical-quantum systems in single-mode fibers (SMFs), capturing the evolution of interference arising from the relevant physical phenomena. We utilize the model to examine frequency allocation in multiband transmission systems, demonstrating that, contrary to common practice of allocating QKD channels in the O-band, increased SKR is achieved by placing quantum channels in the upper E-/lower S-band across the relevant scenarios.
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.
A successful commercial deployment of quantum key distribution (QKD) technologies requires integrating QKD links into existing fibers and sharing the same fiber networks with classical data traffic. To mitigate the spontaneous Raman scattering (SpRS) noise from classical data channels, several quantum/classical coexistence strategies have been developed. O-band solutions place the QKD channel in the O-band for lower SpRS noise but with the penalty of higher fiber loss and can rarely reach beyond 80 km of fiber; another method is C-band coexistence with attenuated classical channels, which sacrifices the performance of classical channels for lower SpRS noise. In this work, a time-interleaving technique is demonstrated to enable the co- propagation of quantum and classical channels in the C-band without sacrificing either performance. By embedding QKD pulses in the gaps between classical data frames, the quantum channel is isolated from SpRS noise in both wavelength and time domains. C-band co- propagation of a polarization-encoding decoy-state BB84 QKD channel with a 100 Gb/s QPSK channel is experimentally demonstrated with quantum bit error rate (QBER) of 1.12%, 2.04%, and 3.81% and secure key rates (SKR) of 39.5 kb/s, 6.35 kb/s, and 128 b/s over 20, 50, and 100 km fibers, respectively. These results were achieved with the presence of classical launch power up to 10 dBm, which is at least one order of magnitude higher than reported works. We also demonstrated the co- propagation of a QKD channel with eight classical channels with total launch power up to 18-dBm (9-dBm per channel), which is the highest power of classical channels reported in C-band coexistence works.
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.
In this paper, we investigate the impact of classical optical communications in quantum key distribution (QKD) over hollow-core fiber (HCF), multi-core fiber (MCF) and single-core fiber (SCF) and propose wavelength allocation schemes to enhance QKD performance. Firstly, we theoretically analyze noise interference in QKD over HCF, MCF and SCF, such as spontaneous Raman scattering (SpRS) and four-wave mixing (FWM). To mitigate these noise types and optimize QKD performance, we propose a joint noise suppression wavelength allocation (JSWA) scheme. FWM noise suppression wavelength allocation and Raman noise suppression wavelength allocation are also proposed for comparison. The JSWA scheme indicates a significant enhancement in extending the simultaneous transmission distance of classical signals and QKD, reaching approximately 100 km in HCF and 165 km in MCF under a classical power per channel of 10 dBm. Therefore, MCF offers a longer secure transmission distance compared with HCF when classical signals and QKD coexist in the C-band. However, when classical signals are in the C-band and QKD operates in the O-band, the performance of QKD in HCF surpasses that in MCF. This research establishes technical foundations for the design and deployment of QKD optical networks.
Continuous-variable quantum key distribution (CVQKD) holds an advantage of well compatibility with classical coherent optical communications. However, there exists a performance trade-off between CVQKD and classical communication on single-mode fiber (SMF) because of the spontaneous Raman scattering. Space-division multiplexing (SDM) technique may provide a feasible way to mitigate this performance trade-off in short-distance communication while CVQKD coexisting with classical signals on few-mode fiber (FMF). Here, we examine the feasibility of CVQKD coexisting with classical signals on FMF and analyze the noise impact in weak coupling regime. We find that the inter-mode crosstalk generated from the mode coupling and re-coupling between modes and the group delay spread originated from the differential group delay (DGD) contribute the main noise sources. DGD may become one of the main limits for FMF-based CVQKD towards high-speed system. In addition, a well channel wavelength management is needed to suppress the inter-mode four-wave-mixing for achieving the positive secret key rates. The numerical simulations identify the key parameters for CVQKD system, enabling a helpful insight for realizing security analysis of the Gaussian modulated coherent state protocol. It shows that CVQKD coexisting with high power classical signals on FMF is feasible to implement with standard telecommunication components and able to operate at higher secret key rates. The results may provide a potential guideline for the practical high-rate CVQKD integrating with the FMF-based configuration.
This paper presents a quantum key distribution (QKD) system that combines classical and quantum signals in a single optical channel for quantum key distribution application. We carefully optimize the communication link by placing the bright classical channel wavelength longer than the weak quantum one, number and specification of wavelength division multiplexers, and maximizing on-off extinction in the intensity modulator to minimize crosstalk of bright classical photons onto the quantum channel. Our experiments demonstrate a secret key rate (SKR) of 4.39 kbps for an emulated channel length of 65.5 km with less than 3% quantum bit error rate (QBER). The experimental study discussed here is relevant for utilization of existing optical fiber networks for QKD, making the system more robust for practical usage.