⚛️ quantum physics

High-Fidelity Quantum Entanglement Distribution in Metropolitan Fiber Networks with Co-propagating Classical Traffic

This paper demonstrates the feasibility of large-scale quantum networking by successfully distributing high-fidelity polarization-entangled photon pairs over up to 100 km of Deutsche Telekom's metropolitan fiber infrastructure, where quantum signals at 1324 nm coexist with standard C-band classical traffic without requiring dedicated fibers or infrastructure changes.

Original authors: Matheus Sena, Mael Flament, Shane Andrewski, Ioannis Caltzidis, Niccolò Bigagli, Thomas Rieser, Gabriel Bello Portmann, Rourke Sekelsky, Ralf-Peter Braun, Alexander N. Craddock, Maximilian Schulz, Kla

Published 2026-03-13

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Matheus Sena, Mael Flament, Shane Andrewski, Ioannis Caltzidis, Niccolò Bigagli, Thomas Rieser, Gabriel Bello Portmann, Rourke Sekelsky, Ralf-Peter Braun, Alexander N. Craddock, Maximilian Schulz, Klaus D. Jöns, Michaela Ritter, Marc Geitz, Oliver Holschke, Mehdi Namazi

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 internet as a massive, bustling highway system. Right now, this highway is packed with heavy trucks carrying your emails, videos, and bank transactions. These are the "classical" signals.

Now, imagine trying to send a fragile, magical message—a "quantum" message—that can only exist if it stays perfectly pure and undisturbed. If a single truck bumps into it, or if the road vibrates too much, the magic disappears.

For a long time, scientists thought you needed a completely separate, empty highway just for these magical messages. But a team of researchers from Deutsche Telekom and Qunnect asked a bold question: "Can we drive our magical quantum cars right alongside the heavy trucks, on the same road, without crashing?"

The answer, according to their new paper, is a resounding YES. They call their project BearlinQ (a nod to Berlin's famous bear, symbolizing strength and resilience).

Here is how they did it, broken down into simple concepts:

1. The "Magic" vs. The "Trucks" (Wavelengths)

Think of light as different colors.

The Trucks (Classical Data): These travel in the "C-band," which is like a deep red color. This is where all your normal internet traffic lives.
The Magic (Quantum Data): The researchers sent their quantum signals in the "O-band," a different color (infrared).

The Analogy: Imagine the highway has two lanes. The trucks are in the right lane, and the magic cars are in the left. Because the colors are different, the trucks don't crash into the magic cars. However, the trucks are so loud (bright) that their noise could still scare the magic cars. The researchers found a way to keep the noise down so the magic cars could drive safely.

2. The "Wobbly Road" (Polarization)

Quantum information is like a spinning top. To read the message, the top needs to be spinning in a specific direction (up, down, left, right).

The Problem: Real-world fiber optic cables are buried underground. The temperature changes, the ground shifts, and trucks drive over them. This makes the "road" twist and turn, causing the spinning top to wobble and lose its direction. If the top stops spinning the right way, the message is lost.
The Solution: They built an automatic stabilizer. Think of it like a self-balancing scooter or a gyroscope in a smartphone. As soon as the road starts to twist, this device instantly twists the light back to the correct position. It does this thousands of times a second, keeping the quantum message steady even as the city of Berlin vibrates around it.

3. The "Magic Trick" (Entanglement)

The core of their experiment is entanglement. Imagine you have two magic coins. No matter how far apart they are—even if one is in Berlin and the other is 60 kilometers away—if you flip one and it lands on "Heads," the other one instantly lands on "Heads" too. They are connected in a way that defies normal physics.

The team successfully sent these "magic coins" (photons) through 60 kilometers of real city fiber, and even tested up to 100 kilometers, while regular internet traffic was flowing right next to them.

4. The Results: A Reliable Network

They didn't just do this for a few minutes in a quiet lab. They ran this system for days, switching between different routes (some short, some long) automatically.

Reliability: The system worked 98.5% of the time. The "downtime" (when the magic stopped working) was less than 1.5%.
Fidelity: This is a score of how "pure" the magic is. They kept the score between 85% and 99%, which is incredibly high for such a long distance.
No New Construction: The best part? They didn't have to dig up the streets to lay new cables. They used the existing cables that Deutsche Telekom already uses for your phone and internet.

Why Does This Matter?

Think of this as the foundation for the "Quantum Internet."

Security: It could lead to unhackable communication (Quantum Key Distribution).
Super-Computing: It could connect quantum computers together to solve problems too big for any single machine.
Sensors: It could link sensors to measure earthquakes or gravity with impossible precision.

The Bottom Line:
This paper proves that we don't need to build a brand-new, expensive "Quantum Highway" from scratch. We can upgrade the existing roads we already have. By using smart hardware to stabilize the signals and keep the noise down, we can start building a future where quantum technology works side-by-side with the internet we use every day. The "BearlinQ" experiment is the proof that this future is ready to be deployed.

1. Problem Statement

The realization of a scalable "Quantum Internet" requires the distribution of entangled photon pairs over long distances. While laboratory demonstrations exist, deploying quantum networks in real-world metropolitan environments faces significant hurdles:

Infrastructure Compatibility: Quantum signals must coexist with existing high-volume classical telecommunications traffic without requiring dedicated fiber strands or major infrastructure overhauls.
Noise and Crosstalk: Classical traffic (typically in the C-band, ~1550 nm) generates noise (specifically spontaneous Raman scattering) that can overwhelm fragile quantum signals.
Environmental Instability: Real-world fiber networks are subject to temperature fluctuations and mechanical stress, causing birefringence changes that rotate photon polarization. Since polarization encoding is sensitive to these drifts, entanglement fidelity degrades rapidly without active compensation.
Scalability and Stability: Previous demonstrations were often limited to single-channel, controlled lab settings, or lacked the long-term stability and dynamic routing capabilities required for commercial operation.

2. Methodology

The authors deployed a hybrid quantum-classical testbed named BearlinQ within Deutsche Telekom's (DT) metropolitan fiber network in Berlin.

Network Architecture:
- Quantum Signal: Entangled photon pairs were generated at 1324 nm (O-band) and 795 nm (NIR). The 1324 nm signal was transmitted over the fiber network, while the 795 nm idler was routed locally for measurement.
- Classical Signal: Conventional bidirectional traffic was transmitted in the C-band (1550 nm).
- Co-propagation Strategy: The choice of O-band for quantum and C-band for classical minimizes Raman scattering noise, as Raman scattering is asymmetric (producing Stokes-shifted photons at longer wavelengths). By placing classical channels at longer wavelengths, the noise falls outside the quantum detection window.
- Hardware Integration: The system utilized Optical Add-Drop Multiplexers (OADMs) to combine and separate quantum and classical channels on the same fiber without dedicated infrastructure.
Key Components:
- Entanglement Source (SRC): A commercial device using warm Rubidium-87 vapor cells to generate bichromatic polarization-entangled pairs via spontaneous four-wave mixing (SFWM). It operates at room temperature with high brightness (>10⁷ pairs/s).
- Active Polarization Stabilization (APC): A critical subsystem comprising an APC-Injector (APC-INJ) and APC-Compensator (APC-CMP). The injector sends a time-multiplexed classical probe signal at the same wavelength as the quantum signal. The compensator analyzes polarization drifts and applies real-time corrections via a low-loss feedback loop to maintain the polarization state.
- Dynamic Routing: A MEMs fiber optical cross-switch allowed dynamic selection of fiber paths ranging from 10 m to 60 km.
Experimental Protocol:
- The system operated continuously over multiple days.
- It alternated between local fiber loops and deployed urban fibers (15 km, 30 km, 60 km, and 100 km).
- CHSH Inequality Tests: Periodic measurements of the Clauser-Horne-Shimony-Holt (CHSH) parameter ( $S$ ) and quantum fidelity ( $F_Q$ ) were performed to verify entanglement.
- Path Switching: The system demonstrated automated switching between paths, triggering the APC to re-align polarization immediately after a switch.

3. Key Contributions

First Metropolitan-Scale Hybrid Deployment: Demonstrated high-fidelity entanglement distribution over a live, operator-managed metropolitan fiber network (Berlin) alongside bidirectional C-band traffic.
Active Stabilization in Real-World Conditions: Proved that active polarization compensation is essential for maintaining entanglement fidelity in buried, uncontrolled urban fibers, achieving stability over days rather than minutes.
Dynamic Routing Capability: Showed that the network can switch between different fiber paths (10 m to 60 km) automatically, with the APC compensating for the new path's polarization characteristics within seconds.
Operational Benchmarking: Established key performance indicators (KPIs) for quantum networks, including downtime metrics, pair rates, and noise tolerance, bridging the gap between academic research and telecom operations.

4. Key Results

Entanglement Fidelity & CHSH Values:
- Achieved Bell-state fidelity bounds between 85% and 99% across all tested distances.
- Measured CHSH $S$ -values between 2.36 and 2.74, significantly exceeding the classical limit of 2, confirming strong quantum nonlocality.
- 100 km Demonstration: Successfully distributed entanglement over ~100 km (97.9 km) with $S = 2.58 \pm 0.03$ and fidelity between 80.5% and 98.4%.
Stability and Downtime:
- The system maintained < 1.5% downtime over extended periods (up to 78 hours for the 100 km loop).
- Compensation Time: The APC corrected polarization drifts with an average time of 511 ms. The 95th percentile for switching from local to 60 km was 9.5 seconds.
- Noise Analysis: Co-propagating classical traffic (at +4 dBm launch power) resulted in a Signal-to-Noise Ratio (SNR) of 35, only slightly lower than the 49 SNR observed without classical traffic, confirming minimal noise pollution.
Pair Rates:
- Delivered coincidence rates ranging from $5 \times 10^2$ to $10^7$ entangled pairs/s after transmission losses, sufficient for practical quantum networking applications.

5. Significance

This work represents a pivotal transition from laboratory-based quantum experiments to commercially viable quantum infrastructure.

Economic Viability: By proving that quantum networks can run on existing fiber assets without new cabling, it drastically reduces the cost and complexity of deployment for telecom operators.
Scalability: The successful integration of OADMs, dynamic switching, and active stabilization provides a practical roadmap for building large-scale, multi-user quantum networks.
Future Applications: The demonstrated reliability and fidelity pave the way for real-world applications such as Quantum Key Distribution (QKD), distributed quantum computing, and quantum sensing, all integrated into the current urban telecommunications backbone.

In summary, the BearlinQ project validates that high-fidelity quantum entanglement can be robustly distributed in a noisy, dynamic, real-world metropolitan environment, establishing the technical foundation for the future Quantum Internet.