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 strict quantum Byzantine agreement with unconditional security to break this bound with nearly $1/2$ fault tolerance due to multiparty correlation provided by quantum digital signatures. Our work strictly obeys the original Byzantine conditions and can be extended to any number of players without requirements for multiparticle entanglement. We experimentally demonstrate three-party and five-party quantum 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.

翻译：拜占庭协议作为区块链的核心基础，旨在使去中心化网络中的每个节点达成共识。经典拜占庭协议不可避免地面临两大问题：一是$1/3$容错极限，即系统要容忍$f$个恶意节点至少需要$3f+1$个节点；二是其经典密码学方法存在的安全漏洞。本文提出一种严格的无条件安全量子拜占庭协议，借助量子数字签名提供的多方关联性，以接近$1/2$的容错率突破这一极限。我们的工作严格遵循原始拜占庭条件，且无需多粒子纠缠即可推广至任意数量的节点。我们通过实验验证了数字账本的三方及五方量子共识。本研究表明了量子在共识问题上的优势，并为量子区块链与量子共识网络开辟了重要方向。