⚛️ quantum physics

Towards National Quantum Communication in Europe: Planning and Sizing Terrestrial QKD Networks

This paper proposes a reproducible planning methodology to estimate the scale and component requirements of national terrestrial Quantum Key Distribution (QKD) networks across Europe, demonstrating the approach with an Austrian case study to derive scaling rules for the EuroQCI framework.

Original authors: Sebastian Raubitzek, Werner Strasser, Sebastian Ramacher, Thomas Lebeth, Andreas Neuhold, Christoph Pacher

Published 2026-04-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sebastian Raubitzek, Werner Strasser, Sebastian Ramacher, Thomas Lebeth, Andreas Neuhold, Christoph Pacher

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

Imagine the European Union is building a super-secure "digital fortress" to protect its most important secrets—government documents, power grid controls, and bank data. They call this project EuroQCI.

The big question the authors of this paper are asking is: "How much building material do we need to construct this fortress for every single country?"

They aren't designing the final building yet (that's for engineers later). Instead, they are acting like architects doing a rough sketch to figure out the scale, cost, and complexity before laying a single brick.

Here is the breakdown of their work using simple analogies:

1. The Problem: The "Too Far to Reach" Dilemma

Quantum Key Distribution (QKD) is a high-tech way to send unbreakable encryption keys. Think of it like sending a glass message in a bottle.

The Catch: If you throw the bottle too far, it might break or get lost. In the real world, the "glass" (the fiber optic cable) has a limit. You can only send a quantum key about 40 to 75 kilometers before the signal gets too weak.
The Solution: To cross a whole country, you need Trusted Repeaters. Imagine these as secure relay stations along a highway. You hand the bottle to Station A, they open it, check it, and re-seal it in a new bottle to send to Station B. As long as you trust the people at the station, the message stays safe.

2. The Experiment: Building a "Virtual Austria"

Since they couldn't build a real network for every country immediately, they created a digital simulation using Austria as a test case.

The Map: They dropped 250 "dots" on a map of Austria. These dots represent important places like ministries, hospitals, and power plants that need protection.
The Rules:
- Every dot needs to be connected to at least two or three other dots (so if one road is blocked, there's a backup).
- They added 50 "Relay Stations" (Trusted Repeaters) to bridge the gaps between distant dots.
- They ran a computer simulation 1,000 times (like rolling dice 1,000 times) to see how the network would look if the dots were placed slightly differently. This ensures their plan is robust and not just a lucky guess.

3. The Results: The "Austrian Blueprint"

The simulation told them exactly what a national network looks like:

The Road Network: They need about 8,600 kilometers of fiber optic cable (that's like driving from Vienna to Berlin and back, three times!).
The Equipment: They need roughly 750 quantum devices (the "bottles" and "seals") and 300 secure computer systems (the "station managers") to handle the keys.
The Hops: Even with relay stations, the longest distance between two stops is kept under 52 kilometers to ensure the signal doesn't break.

4. Scaling Up: The "Cookie Cutter" Method

Once they had the Austrian blueprint, they asked: "How do we make this work for Germany, France, or tiny Luxembourg?"

They created a simple scaling recipe:

More People = More Endpoints: If a country has a big population (like Germany), they need more "dots" (government offices, banks, etc.).
More Land = More Relay Stations: If a country is huge and spread out (like Finland or Sweden), they need more "Relay Stations" to bridge the long distances, even if they don't have that many people.

The Analogy:

The Netherlands is like a small, crowded apartment building. You need many apartments (endpoints) but very short hallways, so you don't need many relay stations.
Finland is like a massive, empty ranch. You don't need as many apartments, but the distances between them are huge, so you need a lot of relay stations to keep the line open.

5. Why This Matters (The "So What?")

The authors emphasize that satellites (space-based QKD) are great for reaching islands or remote areas, but they can't replace the ground network.

The Satellite is the Airplane: It's fast and covers long distances, but it relies on the weather and ground stations.
The Ground Network is the Highway: It's always there, you control it, and it's harder for hackers to attack from the ground.

The paper concludes that Europe needs a hybrid system: a massive, reliable ground highway for the daily traffic, with satellites as a backup for the hard-to-reach spots.

Summary for the Everyday Reader

This paper is a planning guide. It tells European leaders:

"If you want to build a secure quantum internet for your country, here is a rough estimate of how much cable, how many relay stations, and how much money you will likely need. We used Austria as a test kitchen, and now we have a recipe you can use to cook up a plan for any country in Europe."

It's not a construction manual for tomorrow, but it's the blueprint that stops them from building a house that is either too small to fit everyone or too big to afford.

1. Problem Statement

The European Union is developing the EuroQCI (European Quantum Communication Infrastructure) to secure governmental and critical infrastructure communications using Quantum Key Distribution (QKD). While the strategic goal is defined, there is a significant gap in planning-level guidance for national terrestrial QKD networks.

The Core Question: How large must national QKD networks be? Specifically, how many endpoints and trusted repeater nodes are required, what is the total fiber length needed, and how can these metrics be scaled across diverse EU Member States?
The Gap: Existing literature focuses on technological feasibility, specific deployments, or optimization for known demand. There is a lack of reproducible methodologies for early-stage infrastructure dimensioning under uncertain demand and geographic constraints.
Scope: This work focuses exclusively on the terrestrial segment of EuroQCI. It treats space-based QKD as a complementary layer for remote/isolated regions, not a replacement for the terrestrial backbone.

2. Methodology

The authors propose a reproducible planning methodology based on a synthetic network model and Monte Carlo simulation. The approach is demonstrated using Austria as a reference case and then scaled to other EU Member States.

A. The Reference Case (Austria)

Endpoints ( $N_{EP}$ ): 250 synthetic sites representing critical infrastructure (ministries, authorities, data centers). Distribution follows expert assumptions: high density in Vienna, clusters in state capitals, and a rural component.
Topology: Endpoints are connected via a moderately meshed backbone. Each endpoint hosts 2 or 3 QKD links for redundancy.
Distance Constraints: Direct QKD links are limited to 40–100 km (planning target < 75 km) due to optical loss.
Trusted Repeater Nodes ( $N_{TR}$ ): 50 intermediate nodes are introduced to split long distances into feasible QKD spans. These nodes are not demand sites but infrastructure enablers.
Distance Modeling: Geodesic distances are calculated on WGS84 coordinates and multiplied by a detour factor ( $\alpha = 1.5$ ) to approximate real-world fiber routing (following roads/infrastructure).
Components:
- QKD Devices: 750 total (pairs at endpoints and repeaters).
- Key Management: Each site (endpoint + repeater) hosts a Key Management System (KMS) and a Hardware Security Module (HSM).

B. Simulation Framework

The simulation uses a Monte Carlo approach to generate valid network instances:

Endpoint Sampling: Locations are sampled using a "clustered + rural" mixture model within country boundaries.
Graph Construction: An undirected graph is built to satisfy a fixed degree sequence (2 or 3 links per node) while minimizing total edge length.
Optimization: Multiple candidate graphs are generated; the one with the best "robustness score" (minimizing long links and outliers) is selected.
TRN Placement: A greedy algorithm places Trusted Repeater Nodes on the longest edges to ensure all resulting "hops" (QKD spans) remain within the operational limit.
Scaling: The Austrian model is scaled to other EU countries by adjusting:
- Endpoints ( $N_{EP}$ ): Scaled linearly with population.
- Trusted Repeaters ( $N_{TR}$ ): Scaled linearly with land area (to account for distance).

3. Key Contributions

First-Order Planning Framework: Provides the first systematic, reproducible methodology to estimate the scale of national terrestrial QKD backbones across Europe.
Dual-Driver Scaling Model: Establishes that national QKD sizing is driven by two distinct factors:
1. Endpoint Demand: Correlated with population (number of critical sites).
2. Infrastructure Demand: Correlated with geographic area (number of trusted repeaters needed to bridge distances).
Component Estimation: Derives concrete counts for required hardware (QKD devices, KMS, HSMs) and fiber lengths, moving beyond abstract concepts to tangible infrastructure metrics.
Hybrid Architecture Context: Explicitly positions terrestrial QKD as the continuous, physically controllable core, with space-based QKD as a supplement, and advocates for a QKD-PQC hybrid model where QKD secures the backbone and Post-Quantum Cryptography (PQC) handles the "last mile."

4. Key Results

A. Austrian Case Study (Baseline)

Based on 1,000 Monte Carlo runs, the Austrian network requires:

Total Fiber Length: 5,731 km (geodesic) $\rightarrow$ **8,597 km** (route-adjusted).
Hop Lengths:
- Mean: ~22.9 km (adjusted).
- Maximum: ~52.3 km (adjusted).
- Note: These values confirm that without trusted repeaters, many spans would exceed the 75 km operational limit.
Hardware: 750 QKD devices, 300 KMS instances, 300 HSMs.

B. EU-Wide Scaling

The model was applied to all EU Member States (excluding Malta due to size). Key findings include:

Compact States (e.g., Netherlands, Luxembourg): High endpoint counts relative to area, but very short hop lengths and low total fiber requirements.
Large/Geographically Complex States (e.g., Finland, Sweden, France): Require significantly more Trusted Repeaters and longer backbone lengths despite potentially lower population densities.
- Example: Finland requires ~150 TRNs (vs. Austria's 50) due to its vast territory, even with a similar endpoint count.
Maximum Hop Lengths: Across all simulated countries, the route-adjusted maximum hop lengths generally remain within a practical range of 20–95 km, validating the feasibility of the trusted-node architecture.
Table 2 Summary: Provides a comprehensive lookup for planners, listing estimated fiber lengths ( $L_{grid}$ ), mean hop lengths ( $\bar{h}$ ), and maximum hop lengths ( $h_{max}$ ) for every EU member.

5. Significance and Limitations

Significance

Strategic Relevance: Offers immediate, actionable data for EU and national policymakers to assess the cost and scale of EuroQCI terrestrial deployment.
Infrastructure Reality Check: Highlights that terrestrial QKD cannot be replaced by satellites for national backbones due to the need for continuous control, physical inspection, and resilience against space-based threats.
Standardization: Promotes the integration of QKD with classical network layers (SDN, KMS, HSM) as a standard architectural requirement.

Limitations

Synthetic Data: Endpoint locations are generated based on population/area proxies, not actual classified inventories of critical sites.
Simplified Routing: Uses a constant detour factor (1.5) rather than real fiber maps or terrain constraints (e.g., mountains).
No Traffic Modeling: Does not account for specific key consumption rates, bandwidth demands, or service-level agreements.
Exclusions: Excludes cross-border links (though EuroQCI is pan-European) and offshore/island territories in the terrestrial model.
Cost: Provides component counts but not a full techno-economic analysis (CAPEX/OPEX).

Conclusion

The paper concludes that while space-based QKD is valuable for specific use cases, a robust terrestrial backbone remains the strategic core of EuroQCI. The proposed methodology successfully translates high-level security goals into concrete infrastructure estimates, revealing that national QKD networks require a dual scaling strategy: population-driven endpoint density and area-driven repeater density. These results serve as a critical baseline for the next phase of EuroQCI planning, which must move from synthetic models to real-world site inventories and detailed route engineering.