Beating the fault-tolerance bound and security loopholes for Byzantine agreement with a quantum solution

TL;DR

This paper introduces a quantum-based Byzantine agreement framework that surpasses classical fault-tolerance limits and enhances security, demonstrated through experimental consensus in small networks.

Contribution

It presents a quantum digital signature approach that achieves nearly 50% fault tolerance without requiring entanglement, surpassing the classical 1/3 bound.

Findings

Achieved nearly 50% fault tolerance with quantum signatures.

Experimentally demonstrated three-party and five-party consensus.

Framework obeys Byzantine conditions and extends to any number of players.

Abstract

Byzantine agreement, the underlying core of blockchain, aims to make every node in a decentralized network reach consensus. Classical Byzantine agreements unavoidably face two major problems. One is $1/3$ fault-tolerance bound, which means that the system to tolerate $f$ malicious players requires at least $3f+1$ players. The other is the security loopholes from its classical cryptography methods. Here, we propose a Byzantine agreement framework with unconditional security to break this bound with nearly $1/2$ fault tolerance due to multiparty correlation provided by quantum digital signatures. \textcolor{black}{It is intriguing that quantum entanglement is not necessary to break the $1/3$ fault-tolerance bound, and we show that weaker correlation, such as asymmetric relationship of quantum digital signature, can also work.} Our work strictly obeys two Byzantine conditions and can be extended to any number of players without requirements for multiparticle entanglement. We experimentally demonstrate three-party and five-party consensus for a digital ledger. Our work indicates the quantum advantage in terms of consensus problems and suggests an important avenue for quantum blockchain and quantum consensus networks.