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.

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难题：其一是$1/3$容错界限，即系统若要容忍$f$个恶意节点，至少需要$3f+1$个节点；其二是经典密码学方法带来的安全漏洞。本文提出了一种具有无条件安全性的拜占庭协议框架，通过量子数字签名提供的多方关联性，以近$1/2$的容错率突破了这一界限。值得注意的是，突破$1/3$容错界限并不需要量子纠缠，我们证明更弱的关联性（如量子数字签名的不对称关系）同样有效。我们的工作严格满足拜占庭协议的两大条件，可在无需多粒子纠缠的情况下扩展至任意数量的节点。我们通过实验实现了数字账本的三方与五方共识。本研究表明量子在共识问题上的优势，为量子区块链与量子共识网络开辟了重要道路。