⚛️ quantum physics

System-Design Optimization of a Single-Transmitter, Single-Fiber QKD-based ITS Communication Link

This paper proposes and optimizes a single-transmitter, single-fiber architecture for integrated CV-QKD and classical ITS communication, demonstrating that sharing optical infrastructure significantly reduces hardware costs with minimal secure rate loss, while highlighting that increasing receiver bandwidth offers greater performance gains than further spectral separation.

Original authors: Shivang Srivastava, Guillaume Ricard, Yves Jaouën, Matteo Schiavon, Romain Alléaume

Published 2026-10-12

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CC BY 4.0

Original authors: Shivang Srivastava, Guillaume Ricard, Yves Jaouën, Matteo Schiavon, Romain Alléaume

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of secure communication, there is a fundamental tension between safety and efficiency. To protect sensitive data with the highest possible level of security, scientists use a method called quantum key distribution. This technique relies on the laws of physics to generate a secret code that is theoretically impossible to crack without detection. However, to be useful, this secret code must be delivered to a receiver, and the actual message must be sent over the same network. Traditionally, to ensure the delicate quantum signals are not drowned out by the noise of regular data traffic, engineers have built separate physical paths for the two tasks. They run one set of equipment for the secret keys and a completely different set for the messages, effectively doubling the hardware and the fiber optic cables required. This approach is safe but expensive and bulky, making it difficult to deploy widely.

The challenge for researchers has been to find a way to share the same physical infrastructure without compromising the security of the quantum signals. If the strong light used for regular data leaks into the path of the weak quantum signals, it can destroy the secret code. The goal is to pack both the secret key generation and the regular data transmission into a single transmitter and a single fiber optic cable, much like sending two different radio stations over the same wire without them interfering with each other. This requires a delicate balance: the signals must be separated enough to avoid noise, but the system must be efficient enough to make the hardware savings worthwhile.

A team of researchers at Télécom Paris and Inria Saclay in France has tackled this problem by designing and simulating a system that combines these two functions into one streamlined link. They focused on a specific type of quantum communication that uses standard telecommunications components, making it easier to integrate into existing networks. Instead of building two separate systems, they created a model where a single device generates both the quantum signals for the secret keys and the classical signals for the data. These signals travel together down a single fiber optic strand, separated by their frequency, similar to how different colors of light can travel side-by-side without mixing.

The researchers built a detailed computer model to test how well this shared system would perform over various distances. They had to account for several real-world physical effects, such as the way light scatters inside the fiber and the tiny amounts of noise that electronic equipment inevitably adds. Their model calculated the maximum amount of secret, encrypted data that could be sent per second while maintaining the security of the quantum keys. They compared this shared system against a traditional setup where the quantum and classical signals travel on separate fibers with separate transmitters.

The results of their simulation show that sharing the hardware is not only possible but highly efficient over metropolitan distances. At a distance of twenty-five kilometers, which is typical for a city network, the shared system achieved a secure data rate of approximately 12.079 megabits per second. This is almost identical to the 12.095 megabits per second achieved by the traditional, separated system. The difference is so small that it is negligible for practical purposes. The major gain, however, is in the hardware: the shared system requires only one transmitter and one fiber for the direction of travel, whereas the traditional system needs two of each. This effectively cuts the required hardware in half for that direction, significantly reducing the cost and physical footprint of the network.

The study also explored how the system behaves when the distance increases or when the equipment is improved. As the fiber gets longer, the signals weaken, and the noise from the shared data becomes more of a problem. At one hundred kilometers, the shared system still works, but the advantage of sharing becomes slightly more sensitive to the quality of the equipment. The researchers found that the most significant factor for improving the speed of the system was not just separating the signals further apart, but rather improving the receiver's ability to process a wider range of frequencies. When they simulated a receiver with a broader bandwidth, the secure data rate jumped to nearly 24.5 megabits per second, even though the electronic noise increased slightly. This suggests that investing in better receivers yields a much larger return than simply trying to push the signals further apart.

The team also looked at the specific types of noise that occur when signals share a fiber. They found that the main sources of interference are light leaking from the data channel into the quantum channel and a phenomenon called Raman scattering, where the light itself interacts with the glass of the fiber to create noise. Their simulations showed that with careful design, these noise levels remain low enough to keep the system secure. The study confirms that the penalty for sharing the hardware is very small, well within the safety margins required for secure communication.

This work provides a clear path forward for deploying secure quantum networks in real-world cities. It demonstrates that the dream of a single, integrated system that handles both secret keys and regular data is not just a theoretical possibility but a practical reality. By sharing the transmitter and the fiber, network operators can build secure communication links that are far more compact and cost-effective than current solutions. The findings suggest that the future of secure communication lies in integration, where the boundaries between the quantum and classical layers of the network are blurred, allowing them to work together seamlessly on the same physical infrastructure.

Technical Summary: System-Design Optimization of a Single-Transmitter, Single-Fiber QKD-based ITS Communication Link

Problem Statement
Deploying Quantum Key Distribution (QKD) for Information-Theoretically Secure (ITS) communication at scale faces a resource efficiency challenge. Traditional implementations often require duplicated optical hardware (transmitters, fibers, and receivers) to carry both the quantum signal and the necessary classical support traffic (parameter estimation, reconciliation, and OTP-encrypted payloads). While previous studies have established the physical feasibility of co-propagating Continuous-Variable QKD (CV-QKD) and classical signals, a quantitative comparison between the optical hardware savings and the resulting protected user data rate has been lacking. The core problem is to determine if sharing a single transmitter and a single fiber for both classical and Discrete-Modulation CV-QKD (DM-CV-QKD) signals preserves the secure communication rate (SCR) sufficiently to justify the resource reduction, despite the introduction of shared-noise mechanisms like spectral leakage and Raman scattering.

Methodology
The authors propose a system-level optimization framework that couples the physical layer constraints with service-level requirements. The methodology involves:

Architecture Definition: The study models a "shared" architecture where a single transmitter chain generates both classical and quantum signals (using polarization and frequency multiplexing) launched into a single fiber. The receiver and Digital Signal Processing (DSP) chains remain separate for the current platform. This is compared against a "separated" reference architecture where dedicated hardware is used for each signal type.
Secure Communication Rate (SCR) Metric: The authors define SCR as the OTP-encrypted payload rate jointly supported by secret-key generation and the available classical transport capacity. The system is constrained by:
- Key Generation: The secret-key rate ( $K_q$ ) derived from a binomial 64-QAM security bound with a trusted receiver model.
- Classical Transport: The net classical throughput ( $R^c_{net}$ ) must accommodate parameter-estimation disclosure, reconciliation traffic, and the OTP payload.
- Noise Models: The model incorporates spectral leakage (classical power leaking into the quantum band), forward Raman scattering, and bandwidth-dependent trusted electronic noise.
Optimization Process: A nested optimization is performed over variables including fiber length ( $L$ ), classical and quantum symbol rates ( $f_c, f_q$ ), center-frequency separation ( $\Delta f$ ), modulation variance ( $V_A$ ), and classical launch power ( $P_c$ ). The objective is to maximize the SCR subject to physical constraints (e.g., classical SNR $\ge$ 14 dB) and hardware resource limits.
Resource Comparison: The study utilizes an "equal-weight hardware index" ( $C^{(1)}_{hw} = N_{Tx} + N_{fib} + N_{Rx/DSP}$ ) to normalize rates against hardware counts. A shared architecture is considered beneficial if its rate-per-resource ratio meets a specific threshold (1.761 dB for the modeled one-direction scenario) compared to the separated reference.

Key Contributions

Service-Level Formulation: The paper formulates SCR as the joint OTP payload rate supported by key generation and classical transport, explicitly accounting for the classical overhead required by QKD protocols.
Integrated Physical Model: It combines a binomial 64-QAM security bound, a trusted receiver model, and physical noise terms (leakage, Raman, and electronic noise) into a unified framework for system design.
Joint Optimization: The study jointly optimizes quantum and classical parameters (symbol rates, frequency separation, launch power) and examines the trade-offs between spectral separation, receiver bandwidth, and distance-dependent noise.

Results

Metropolitan Distance Performance: At a fiber length of 25 km, the optimized shared link achieves an SCR of 12.079 Mbps, compared to 12.095 Mbps for the separated-link reference. This represents a negligible rate loss (penalty of ~0.006 dB) while reducing the transmitter and fiber counts from two to one in the modeled direction.
Hardware Efficiency: The shared architecture reduces the hardware index from 6 (separated) to 4 (shared). The resulting rate-per-resource ratio significantly exceeds the threshold required to justify the integration, with a hardware-normalized rate of ~1,381 kbit/s compared to ~170 kbit/s for earlier OTP systems cited in the literature.
Distance and Power Scaling: As distance increases from 25 km to 100 km, the required classical launch power increases to maintain the SNR, which in turn increases Raman noise. At 100 km, the SCR drops to 0.205 Mbps, and the sharing penalty rises to 0.444 dB, remaining below the 1.761 dB threshold.
Receiver Bandwidth Impact: Increasing the receiver bandwidth budget from 350 MHz to 750 MHz (despite higher trusted electronic noise) raises the projected 25-km SCR to 24.535 Mbps. The study finds that increasing usable receiver bandwidth provides a greater benefit than further spectral separation once leakage is suppressed.
Noise Sensitivity: The system is sensitive to out-of-band transmitter noise. At 100 km, an additional noise fraction of -60 dB relative to the launch power causes the sharing penalty to exceed the hardware-normalization threshold, whereas at 25 km, the system tolerates higher noise levels.

Significance and Claims
The paper claims that sharing the transmitter and fiber for DM-CV-QKD and classical signals is a viable integration path for metropolitan-scale ITS communication. The primary significance lies in demonstrating that optical resource savings (reducing transmitters and fibers by 50% in one direction) can be achieved with minimal rate loss, provided that:

Unwanted classical power in the quantum receive band is limited through adequate spectral separation and polarization extinction.
Receiver bandwidth is optimized to balance symbol rate gains against electronic noise.

The authors conclude that for the studied metropolitan link, sharing saves optical resources effectively. They suggest that future work should extend this co-design framework to include composable finite-size security proofs and further integration of receiver/DSP chains to reduce the remaining hardware index. The results serve as a quantitative basis for moving from physically separated QKD and classical links to integrated, resource-efficient architectures.

Technical Summary: System-Design Optimization of a Single-Transmitter, Single-Fiber QKD-based ITS Communication Link

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