Experimental demonstration of scalable quantum blockchain with exponentially superior quantum communication complexity

Quantum blockchains offer a path to information-theoretic security that surpasses the limitations of classical distributed ledgers, which are vulnerable to quantum computing attacks and restricted by the one-third fault-tolerance bound. However, existing quantum consensus protocols often suffer from the blockchain trilemma, forcing a choice between exponential communication complexity or the use of fragile, experimentally demanding multipartite entanglement. This paper presents a scalable alternative that avoids these pitfalls.

The researchers implement a circular quantum Byzantine agreement (QBA) protocol. Unlike previous schemes that require complex entanglement, this approach uses weak coherent states and a semi-decentralized architecture. A central certificate authority (CA) is utilized specifically for signature verification, which significantly simplifies the network topology. The protocol operates in three phases: order distribution, circular gathering, and consensus output, ensuring that all honest participants reach an identical, authenticated agreement on transaction sequences.

• Scalability: The protocol achieves a communication complexity of O(N^2), representing an exponential improvement over previous fully decentralized quantum consensus schemes.
• Experimental Performance: The system was deployed over a metropolitan fiber network in Nanning, China, connecting five user nodes and one CA. It successfully tolerated up to three malicious nodes, the highest fault tolerance demonstrated to date in such a system.
• Practical Application: By integrating this consensus layer with a smart-contract-based application layer, the team demonstrated a functional token exchange system. The platform achieved a sustained throughput of 805.3 transactions per second with zero failures, proving the feasibility of quantum-secured decentralized finance.