High-key-rate Fully-Passive Quantum Access Network with Thermal Source

This paper presents and experimentally demonstrates a record-breaking 19.48 Mbps downstream fully-passive quantum access network using passive state preparation, which extends point-to-point protocols to point-to-multi-point architectures while remaining compatible with existing classical optical infrastructure.

The Gist

Imagine you want to send a super-secret message to a group of friends, but you're worried that a spy (let's call her "Eve") might be listening in. In the world of quantum physics, there's a special way to do this called Quantum Key Distribution (QKD). It's like creating a secret code that is mathematically impossible to crack without being detected.

For a long time, building a network to send these secret codes to many people at once (like a neighborhood or an office building) was like trying to run a high-speed train on a bumpy, old dirt road. It was expensive, complicated, and slow.

This paper introduces a new, smoother road: a Fully Passive Quantum Access Network. Here is how it works, broken down into simple concepts:

1. The Old Way vs. The New Way

• The Old Way (Active Modulation): Imagine a mail carrier who has to stop at every house, manually stamp a letter, and then run to the next one. In the old quantum networks, the sender had to use fast, expensive electronic "stamps" (modulators) to change the quantum signals for every single user. This was slow, generated heat, and was hard to keep stable.
• The New Way (Passive State Preparation): The authors propose a different approach. Instead of the sender manually stamping every letter, they use a thermal source (like a special light bulb that naturally flickers in a random, quantum-safe way). They split this natural light into many paths using simple mirrors and splitters.
  • The Analogy: Think of it like a fountain. Instead of a gardener spraying water at specific people one by one, the fountain sprays water naturally in all directions. The "passive" part means no one is actively forcing the water to go where it needs to go; it just flows naturally, and the receivers catch what comes their way.

2. The "Hybrid" Highway

The team built a test network that combines two types of roads:

• The Fiber Road: A 5-kilometer underground cable (like the internet cables in your walls).
• The Air Road: A short stretch of open air (like sending a message from a window to a neighbor's window).
  They mixed these two to simulate a real-world scenario where the signal travels through a building's wiring and then through the air to a mobile device or a home.

3. The "One-to-Many" Magic

The biggest breakthrough here is that this system works for one sender talking to many receivers (a "Point-to-Multi-Point" network).

• In the past, these passive systems only worked for one-on-one conversations.
• In this experiment, the sender (Alice) sent the secret signal to four different people (Bobs) at the same time.
• Because the system is "passive," it doesn't lose speed or quality just because it's splitting the signal among four people. It's like a radio station broadcasting to many cars; the signal doesn't get weaker just because more cars are listening.

4. The Results: Speed and Stability

The team achieved something record-breaking:

• Speed: They generated secret keys at a rate of 19.48 Megabits per second for each person. To put that in perspective, previous similar experiments were often thousands of times slower. It's the difference between dial-up internet and high-speed fiber.
• Stability: Even with the signal traveling through both fiber and open air (which can be tricky due to wind or temperature), the system stayed stable.
• Compatibility: The best part? This new system speaks the same "language" as the existing internet cables in our homes. You wouldn't need to rip out your current wiring to install this; it fits right into the existing infrastructure.

5. Why This Matters

The paper claims this is a major step toward making quantum security available for everyday use, specifically for:

• Local Area Networks (LANs): Securing a home or office network.
• Mobile Terminals: Securing connections for devices like phones or tablets.

In a nutshell: The authors built a quantum network that uses a "natural" light source instead of expensive, complex machinery. This allows them to broadcast ultra-secure keys to multiple users simultaneously at incredibly high speeds, using a setup that fits perfectly into the internet cables we already have. It's a faster, cheaper, and simpler way to secure the "last mile" of our digital connections.

Technical Summary

1. Problem Statement

Quantum Access Networks (QANs) are critical for the "last mile" deployment of Quantum Key Distribution (QKD). However, existing QAN architectures face significant limitations:

• Active Modulation Bottlenecks: Traditional Continuous-Variable QKD (CVQKD) networks rely on active protocols (e.g., Gaussian-Modulated Coherent States - GMCS) requiring high-speed amplitude and phase modulators. Achieving high extinction ratios, linear modulation, and stability at high speeds is challenging, costly, and complex to integrate.
• Scalability vs. Rate Trade-off: Previous Passive State Preparation (PSP) CVQKD protocols were limited to Point-to-Point (PTP) architectures. Extending these to Point-to-Multi-Point (PTMP) networks without sacrificing key rates or security has been a critical gap.
• Infrastructure Compatibility: Many quantum networks require dedicated infrastructure, making integration with existing classical optical access networks difficult.

2. Methodology

The authors propose and experimentally demonstrate a Passive-State-Preparation Quantum Passive Optical Network (PSP-QPON).

• Core Protocol (PSP):
  • Instead of active modulation, the system uses a thermal source (Amplified Spontaneous Emission - ASE) to generate quantum states.
  • The source signal is split: one part is sent to users (Bobs), and the other is measured locally by the source (Alice) via heterodyne detection.
  • Alice's local measurement provides an optimal estimation of the outgoing state, treating the uncertainty as additive excess noise (PSP noise). This eliminates the need for active modulators at the source.
• Network Architecture (PTMP):
  • The network extends PSP from PTP to a 1-to-4 Point-to-Multi-Point configuration.
  • It utilizes a passive optical splitter to distribute signals to multiple Quantum Network Units (QNUs).
  • Security Model: The protocol ensures that each QNU generates a unique key. Through reverse reconciliation, the source generates independent keys with each user. The security analysis accounts for potential eavesdropping (Eve) and ensures that correlations between different Bobs do not leak information (assuming Bobs are trustworthy).
• Experimental Channel:
  • A hybrid channel was constructed to simulate a real-world Local Area Network (LAN):
    • 5 km Single-Mode Fiber: Simulates the "fiber-to-the-home" link.
    • Free-Space Segment: A 1-meter indoor free-space link using collimators to simulate Li-Fi or short-range wireless access.
  • The total average attenuation for the hybrid channel was -10.96 dB.
• Signal Processing & Calibration:
  • Real-time Shot Noise Acquisition: An optical switch alternates between signal and shot noise calibration to normalize data against laser intensity fluctuations.
  • Advanced DSP: The system employs a machine learning-based signal demodulation algorithm using a Gated Recurrent Unit (GRU) with a local adaptive attention mechanism. This replaces traditional training frames, enabling efficient frame synchronization and phase compensation without active modulation overhead.

3. Key Contributions

• First PTMP PSP-CVQKD Demonstration: Successfully extended the PSP protocol from point-to-point to a scalable point-to-multi-point network, breaking the PTP bottleneck of previous thermal-source QKD systems.
• Record-Breaking Key Rates: Achieved a secure key rate of 19.48 Mbps per user in a 1-to-4 network configuration. This is significantly higher than previous CV access network experiments (which typically range from kbps to low Mbps).
• Fully Passive Architecture: Eliminated the need for high-speed active modulators at the source, reducing cost, complexity, and power consumption while maintaining high stability.
• Infrastructure Compatibility: The system operates at 1550 nm and is fully compatible with existing classical optical access networks, allowing for seamless integration without modifying current fiber infrastructure.

4. Experimental Results

• Key Generation Rate:
  • Per User: 19.48 Mbps (highest reported for a PTMP CVQPON).
  • Average Asymptotic SKR: 19.29 Mbps over an equivalent 20 km fiber channel.
• Performance Metrics:
  • Reconciliation Efficiency: 96%.
  • Detection Efficiency: 0.56.
  • Excess Noise: Average excess noise across the four users was approximately 0.05 SNU (Shot Noise Units), indicating high system stability.
  • Mutual Information: The mutual information between different Bobs ($I_{B_i B_j}$) was negligible (max 0.0039 bit/pulse), far below the Holevo bound, confirming that user correlations do not compromise security.
• Comparison: The achieved rate is orders of magnitude higher than recent state-of-the-art CV access networks (e.g., 12.05 Mbps over 15km in 2025, 1.01 Mbps over 21km in 2024).

5. Significance

• Scalability for Quantum Internet: This work provides a viable, high-rate, and cost-effective solution for large-scale QKD deployment in local area networks (LANs), such as smart homes, mobile terminals, and enterprise networks.
• Technological Leap: By combining passive state preparation with passive optical networking, the authors demonstrate that high-speed quantum communication does not require expensive, complex active modulation hardware.
• Practical Deployment: The use of hybrid fiber-free-space channels and compatibility with classical infrastructure paves the way for immediate integration into existing telecommunications networks, accelerating the transition from experimental QKD to commercial quantum-secure services.
• Future Outlook: The paper identifies areas for improvement, such as using polarization-maintaining components to reduce attenuation and implementing local local oscillator (LLO) schemes to further simplify the receiver end, setting a roadmap for next-generation quantum access networks.