  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.
