QUROPE Quantum Information Processing and Communication in Europe

Bit commitment is a fundamental cryptographic primitive with numerous applications. Quantum information allows for bit commitment schemes in the information theoretic setting where no dishonest party can perfectly cheat. The previously best-known quantum protocol by Ambainis achieved a cheating probability of at most 3/4, while Kitaev showed that no quantum protocol can have cheating probability less than 1/sqrt{2}. Closing this gap has since been an important and open question. In this paper, the authors provide the optimal bound for quantum bit commitment.