Quantum bit commitment with a composite evidence

Abstract

Entanglement-based attacks, which are subtle and powerful, are usually believed to render quantum bit commitment insecure. We point out that the no-go argument leading to this view implicitly assumes the evidence-of-commitment to be a monolithic quantum system. We argue that more general evidence structures, allowing for a composite, hybrid (classical–quantum) evidence, conduce to improved security. In particular, we present and prove the security of the following protocol Bob sends Alice an anonymous state. She inscribes her commitment b by measuring part of it in the + (for b = 0) or × (for b = 1) basis. She then communicates to him the (classical) measurement outcome Rx and the partmeasured anonymous state interpolated into other, randomly prepared qubits as her evidence-of-commitment.