  A new interactive quantum zero-knowledge protocol for identity authentication
implementable in currently available quantum cryptographic devices is proposed
and demonstrated. The protocol design involves a verifier and a prover knowing
a pre-shared secret, and the acceptance or rejection of the proof is determined
by the quantum bit error rate. It has been implemented in modified Quantum Key
Distribution devices executing two fundamental cases. In the first case, all
players are honest, while in the second case, one of the users is a malicious
player. We demonstrate an increase of the quantum bit error rate around 25% in
the latter case compared to the case of honesty. The protocol has also been
validated for distances from a back-to-back setup to more than 60 km between
verifier and prover. The security and robustness of the protocol has been
analysed, demonstrating its completeness, soundness and zero-knowledge
properties.
