Researchers at KTH Royal Institute of Technology in Stockholm, Sweden have demonstrated sustained quantum key distribution (QKD) over a fiber link for one month. This test forms part of the National Quantum Communication Infrastructure in Sweden, the Swedish chapter of a larger European effort to evaluate practical QKD deployment. The work details procedures for characterizing fiber links and relating measurable properties to key factors affecting the secret key rate, confirming the feasibility of QKD within the existing infrastructure. This research supports the EU’s EuroQCI initiative, begun in 2023, to build a continent-wide quantum communication network.

Swedish NQCIS Deployment: Evaluating Real-World QKD Performance

Evaluations of fiber optic links within the Swedish National Quantum Communication Infrastructure in Sweden (NQCIS) connected Stockholm through a network designed as an open testbed for quantum key distribution (QKD). The NQCIS, launched in 2023, is the Swedish chapter of the broader EuroQCI initiative and involved a consortium including KTH, Royal Institute of Technology, Stockholm University, Chalmers Technical University, Linköping University, Ericsson AB, Quantum Scopes AB and quCertify AB.

Studies focused on characterizing these links to determine their suitability for QKD deployment and to relate measurable properties to factors influencing the secret key rate. Researchers established a simplified model to connect physical parameters of the fiber links to the resulting secret key rate, aiming to identify essential elements of QKD performance.

Analysis centered on four representative links within the NQCIS network, assessing contributions to the secret key rate from signal attenuation, background noise and deterioration of quantum states due to the fiber channel. This work details techniques for measuring these parameters and provides guidelines for planning and evaluating future QKD deployments, an important step toward practical quantum communication networks.

A one-month long test confirmed the feasibility of QKD operation on one of the deployed links, demonstrating sustained functionality beyond initial proof-of-concept demonstrations. The study considered link length as a primary factor, alongside contributions from attenuation, background noise and quantum state deterioration, all impacting the achievable secret key rate.

The team’s analysis highlights the importance of cross-checking link and system parameters, and optimizing them to ensure acceptable QKD performance levels. “We have reported here many of the physical parameters that are relevant for the deployment of QKD on a fiber network,” the paper states, “and have shown how they can be used to predict the performance of a QKD system.” The research extends beyond simply achieving a functional QKD link; it provides a framework for predicting performance based on measurable fiber characteristics.

By relating parameters like signal attenuation and background noise to the secret key rate, the team offers a practical approach to optimizing QKD in existing infrastructure. This focus on linking physical measurements to key performance indicators is intended to guide future deployments and ensure the reliable operation of quantum communication networks across Europe and beyond.

Fiber Link Characterization for Compatible QKD Systems

Fiber link tortuosity, a measure of how much a fiber deviates from a straight path, varied significantly across the Swedish National Quantum Communication Infrastructure, reaching a maximum of 2.37 for the M2 link. This indicates that metropolitan fiber networks in the area are constructed from numerous short segments, a design choice prioritizing reconfigurability and service to densely populated areas. Conversely, longer links traversing less populated regions exhibited lower tortuosity, suggesting a more direct routing along major transportation corridors like highways.

The distribution of optical signal peaks also differed between links, a finding consistent with the arbitrary allocation of optical channels within the network. These variations highlight the need for careful channel selection and optimization to minimize signal loss and maximize the performance of quantum key distribution (QKD) systems.

Polarization-mode dispersion (PMD), a phenomenon that can limit bandwidth in optical systems, was also characterized. PMD arises from differing propagation speeds of polarization modes within the fiber, causing temporal broadening of light pulses and reducing signal purity. Countermeasures against PMD, such as inserting strongly birefringent segments, are available to compensate for these effects and maintain signal integrity. The characterization of PMD is common in the telecom industry, but it is important for quantum optics applications requiring precise polarization control.

Simplified Model Relating Fiber Properties to Secret Key Rate

The model allows for prediction of secret key rates based on measurable characteristics of fiber optic links, streamlining the process of evaluating suitability for QKD systems. This approach is critical as Europe expands its quantum communication networks under initiatives like EuroQCI. The research team focused on an analytical model expressing the secret key rate as a function of realistic parameters, specifically the BB84 protocol, a widely used discrete-variable QKD scheme, to be a foundational archetype for related protocols.

These equations, as detailed in their work, allow for a more efficient assessment of QKD potential. The model breaks down the overall efficiency into components representing the link and the system, and similarly separates background noise and quantum bit error rates. This modular approach, represented graphically in the published work, enables targeted optimization of each element to maximize the secret key rate.

This characterization is not merely academic. It offers a practical pathway for selecting compatible links and devices, ensuring requirements are met in real-world QKD implementations.

BB84 Protocol Implementation with Decoy States for QKD

Attenuation of laser pulses approximating single-photon states is central to the successful implementation of the BB84 protocol, and the use of decoy states, varying signal, decoy and vacuum intensities, protects against photon-number-splitting attacks during key distribution. The research team focused on a simplified model prioritizing clarity over complete realism, aiming to isolate the core factors influencing quantum key distribution performance. This approach allows for a focused evaluation of how key parameters interact, rather than being obscured by complex system details.

The team developed equations to express the secret key rate (SKR) as a function of total transmittance (η), the probability of vacuum pulse detection (Y_0), and the intrinsic quantum bit error rate (q). These equations reveal that the SKR decreases gradually if q and $Y_{0} / η$ remain below 1%, but rapidly approaches zero when they approach maximum tolerable values of approximately 4.6% and 0.1, respectively.

These thresholds, while specific to the chosen decoy protocol and signal intensity, demonstrate a general principle: overoptimizing q or $Y_{0}$ yields diminishing returns, and maintaining low values is paramount. The methodology itself is generalizable, allowing recalculation of thresholds for different intensity levels. The team’s work provides a pathway for selecting compatible links and optimizing QKD system components for improved performance.

Key Rate Equation: SKR Calculation with Error and Gain Parameters

Researchers developed an equation defining SKR as: SKR = $Q_{1} (1 - h (E_{1})) - Q_{μ} h (E_{μ})$ , where $Q_{1}$ represents single-photon gain and $E_{1}$ is the associated phase error rate, while $Q_{μ}$ and $E_{μ}$ define overall signal gain and bit error rate respectively. This formulation allows for a detailed assessment of how these parameters interact to influence QKD performance. The study further refined these parameters, establishing specific relationships between measurable link characteristics and SKR.

Single-photon gain (Q_1) is calculated as e^(-1)(η + $Y_{0} - η$ Y_0), with phase error rate (E_1) defined as (qη + 1/2Y_0)/(η + $Y_{0} - η$ Y_0), where η represents total transmittance and $Y_{0}$ is the probability of a vacuum pulse triggering detection. These equations demonstrate that SKR is directly proportional to η, indicating that maximizing transmittance is important for optimal performance.

Threshold Detector Modeling in QKD Systems

Characterizing polarization-mode dispersion (PMD) is a common practice in telecommunications to maximize bandwidth, but its impact on quantum key distribution (QKD) systems requires specific analysis. This technique allows for a detailed understanding of how the fiber link alters quantum signals, informing strategies to mitigate signal degradation and maintain secure communication. Automated alignment procedures, used when the quantum bit error rate (QBER) exceeds acceptable limits, do not consistently achieve optimal performance, resulting in a relatively high QBER of 4.2% on the key basis.

This limitation may stem from systematic errors or complex interactions between correction systems and unmodeled link parameters, suggesting that current models require refinement to accurately predict real-world QKD performance. The simplified model used in this work, while effective, does not account for finite-size effects, imperfect error correction algorithms, detector imperfections like dead time or afterpulsing, or engineering constraints on intensity levels.

The quick decline in secret key rate (SKR) at high attenuation levels is primarily due to background noise dominating the detected signal, causing the QBER to exceed the threshold for secure key generation. Quantifying these parameters is important for predicting deployment performance and establishing quantitative requirements for drift compensation systems and QKD pulses, ensuring the long-term stability and security of quantum communication networks.

One-Month QKD Test Verifying Feasibility on Swedish Links

Characterizing fiber links is essential for deploying quantum key distribution (QKD) systems, and the Swedish chapter of the European initiative has focused on establishing procedures for assessing link suitability. This extended trial period provides confidence in the stability and reliability of QKD over Swedish fiber infrastructure. Researchers developed a simplified model to capture the essence of QKD performance, requiring minimal information about the QKD devices themselves.

This model allows for evaluation of QKD feasibility on any link, streamlining the process of assessing compatibility between existing infrastructure and quantum communication technologies. This methodology is not limited to specific protocols or links, offering a generalizable approach to optimizing QKD deployments.

NQCIS Consortium: Building Swedish Quantum Communication Expertise

Launched in 2023, this partnership aimed to procure, deploy, and evaluate a network of commercial quantum key distribution (QKD) devices connected by optical fiber centered in Stockholm, establishing an open testbed for QKD technology within the country. This collaborative effort has demonstrably built Swedish expertise in quantum communication, connecting academic and industrial stakeholders to prepare for future advancements in practical QKD systems. Beyond research results currently in preparation and already published, NQCIS addressed initial deployment challenges common to establishing QKD networks, including the characterization of polarization-mode dispersion (PMD).

This approach allows for cross-checking link properties with QKD device specifications, ensuring compatibility and optimizing performance. The NQCIS consortium’s efforts have not only validated QKD functionality over Swedish fiber infrastructure but also established a foundation for sustained growth in quantum communication expertise within the nation.

👉 More information
🗞 Deploying QKD on the fiber links of the National Quantum Communication Infrastructure in Sweden
✍️ Giulio Foletto, Mira Abu Yahia, Shreya Gouravaram Navalur, Carlos Brotons Montoro, Hilma Karlsson, Dena Karunianto Wibowo, Pascal Lefebvre and Katia Gallo
🧠 ArXiv: https://arxiv.org/abs/2611.12279v1

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