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Experimental demonstration of a reconfigurable three-user quantum digital signature network

This paper presents the experimental demonstration of a reconfigurable three-user quantum digital signature network over fiber optics, where any participant can dynamically act as a signer using identical hardware, successfully generating valid signatures even in regimes where quantum key distribution fails.

Original authors: Jeong Youn-Chang, Kim Seok, Hong Changho, Ji Se-Wan

Published 2026-10-13

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CC BY 4.0

Original authors: Jeong Youn-Chang, Kim Seok, Hong Changho, Ji Se-Wan

Original paper licensed under CC BY 4.0 (https://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 digital world, proving that a message is truly from who it claims to be, and that it has not been altered, is the job of digital signatures. For decades, these electronic seals have relied on complex mathematical puzzles that are hard for today's computers to solve. However, the rise of powerful quantum computers threatens to crack these puzzles, potentially leaving our digital communications vulnerable. To counter this, scientists are turning to the laws of physics themselves. One such approach is quantum key distribution, a method that uses particles of light to create secret codes that are theoretically impossible to intercept without detection. While this technology is excellent for keeping messages private, a related but distinct technology called quantum digital signature aims to do something different: it guarantees the origin and integrity of a message without necessarily keeping the content secret. The challenge has been that early versions of these signature systems were rigid, requiring fixed roles for the people involved, which made them difficult to use in a flexible, real-world network where anyone might need to send a message.

A team of researchers has now demonstrated a way to make these quantum signature networks flexible and practical. Working with a network of three participants connected by fiber-optic cables, they built a system where any of the three people can step into the role of the sender, or signer, at any time. In their setup, the participants are linked through a central switch that directs the flow of quantum states, which are the specific patterns of light used to carry information. When one person acts as the signer, the other two act as the receiver and the verifier. The unique feature of this experiment is that the hardware at each location is identical. Because every node is built the same way, the system can dynamically reassign who is signing a message simply by changing how the switch routes the light, without needing to rebuild or reconfigure the equipment. This proves that a quantum network can be adaptable, allowing any participant to initiate a secure signature process.

The researchers tested this reconfigurable network by having each of the three participants take turns being the signer. In every scenario, the system successfully generated valid signatures. They measured how fast these signatures could be created and found that the speed varied depending on which two people were communicating, largely due to differences in how efficiently their detectors could catch the incoming light particles. The team also compared their results to the performance of standard quantum key distribution, the technology used to create secret keys for encryption. They discovered a crucial advantage for signatures: the system could generate valid signatures even when the conditions were too poor for the other technology to produce a secret key. This happens because creating a signature does not require the same level of error correction and data processing as creating a secret key.

This difference is significant for the future of quantum communication. In the experiment, the researchers found that when they tried to create a secret key using the small amount of data available for a single signature, the process failed because the information lost during error correction was too high. However, the signature system succeeded with that same small amount of data. This suggests that quantum signatures can operate with lower delays and require less memory storage than systems designed solely for secret keys. By showing that a network can switch roles on the fly and still maintain security, even in challenging conditions, the researchers have taken a vital step toward building robust quantum communication infrastructures where flexibility and reliability go hand in hand.

Technical Summary: Experimental Demonstration of a Reconfigurable Three-User Quantum Digital Signature Network

Problem Statement
Quantum Digital Signatures (QDS) offer information-theoretic security (ITS) for integrity, non-repudiation, and data origin authentication, addressing the vulnerabilities of classical public-key schemes to quantum adversaries. However, existing QDS implementations typically assign fixed roles (signer, authenticator, and verifier) to network participants. This rigidity limits their applicability in practical quantum networks where dynamic role assignment is essential. Furthermore, while QDS and Quantum Key Distribution (QKD) both rely on quantum states, they have distinct operational requirements; QKD requires large data blocks for parameter estimation to distill secret keys, whereas QDS aims to prevent forgery and repudiation using correlated data. The challenge lies in demonstrating a flexible QDS network that can dynamically reconfigure roles and operate effectively at small post-processing block sizes where traditional QKD fails to generate a secret key.

Methodology
The authors implemented a three-user QDS network using fiber-optic links connecting three nodes: Alice, Bob, and Charlie. The system is built upon the protocol proposed by Amiri et al. (2016), which utilizes time-bin encoding and does not require long-term quantum memory or entanglement swapping.

Network Architecture: A network switch composed of three 3-port optical circulators and a 2×2 optical switch enables dynamic routing. This design allows any node to act as the signer, authenticator, or verifier. When a node acts as the signer, it receives quantum states from the other two participants.
Optical Setup: All nodes utilize identical hardware for encoding and decoding. The system employs a picosecond pulsed laser, intensity modulators, and phase modulators to generate four time-bin quantum states. The protocol uses a two-decoy-state method (signal, decoy 1, and decoy 2/vacuum) to estimate channel parameters.
Protocol Execution:
1. Distribution Stage: The signer receives quantum states from the other two participants to establish raw keys. The authenticator and verifier perform QKD between themselves to generate a shared secret key for the symmetrization step.
2. Messaging Stage: The signer signs a message and sends it to the authenticator. The authenticator verifies the signature against their symmetrized key and forwards the message to the verifier for final validation.
Security Analysis: The study employs finite-key security analysis to determine the required signature length ( $L$ ) and threshold error rates ( $s_a, s_v$ ) based on observed Quantum Bit Error Rates (QBERs). The analysis accounts for unforgeability, non-repudiation, and transferability, bounding failure probabilities for robustness, repudiation, and forging.

Key Contributions

Reconfigurable Network: The work demonstrates the first experimental realization of a QDS network where any participant can dynamically assume the role of the signer without hardware reconfiguration.
Operational Flexibility: By using identical node designs and a switch-based routing mechanism, the system successfully generated valid signatures for all possible signer configurations (Alice, Bob, or Charlie).
Small Block Size Operation: The study highlights a fundamental operational difference between QDS and QKD. It demonstrates that QDS can generate valid signatures using small classical post-processing block sizes (corresponding to the signature length $L$ ), a regime where QKD fails to produce a secret key due to excessive information disclosure during error correction.

Results

Performance Metrics: The system achieved raw key rates ranging from approximately 10 to 40 kbps depending on the sender-receiver pair. Signature rates varied based on the required signature length $L$ , which was calculated via finite-key analysis for each configuration.
Asymmetry: Despite identical channel transmittance in opposite directions, raw key rates showed asymmetry (e.g., Bob-to-Alice was higher than Alice-to-Bob), attributed to differences in detection efficiency at the receiving nodes.
QKD vs. QDS Comparison:
- When using a large block size ( $10^6$ bits) for QKD post-processing, secret keys were successfully generated between the authenticator and verifier for all configurations.
- When using a block size corresponding to the signature length ( $L+k$ ), QKD failed to generate secret keys for certain links (specifically Bob-Charlie and Charlie-Bob) because the information leaked during error correction exceeded the shared secret information.
- In contrast, the QDS protocol successfully generated valid signatures in these same small-block regimes.
Security Parameters: The system achieved repudiation and forging failure probabilities on the order of $10^{-5}$ , with signature lengths ( $L$ ) ranging from $8.5 \times 10^4$ to $1.6 \times 10^5$ bits depending on the signer configuration.

Significance
The paper claims that this work establishes the feasibility of reconfigurable quantum signature networks, representing a step toward scalable and robust quantum communication infrastructures. The primary significance lies in demonstrating that QDS offers a practical advantage over QKD in specific network scenarios: it can operate with lower latency and reduced memory requirements by functioning effectively with small data blocks where key distillation is not achievable. This suggests that QDS is a viable, lightweight solution for future quantum networks requiring rapid, secure authentication without the heavy resource overhead of full QKD key generation.

Technical Summary: Experimental Demonstration of a Reconfigurable Three-User Quantum Digital Signature Network

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