Quantum solution to the Byzantine agreement problem.

We present a solution to an old problem in distributed computing. In its simplest form, a sender has to broadcast some information to two receivers, but they have access only to pairwise communication channels. Unlike quantum key distribution, here the goal is not secrecy but agreement, and the adversary (one of the receivers or the sender himself) is not outside but inside the game. Using only classical channels this problem is provably impossible. The solution uses pairwise quantum channels and entangled qutrits.