  Protecting secrets is a key challenge in our contemporary information-based
era. In common situations, however, revealing secrets appears unavoidable, for
instance, when identifying oneself in a bank to retrieve money. In turn, this
may have highly undesirable consequences in the unlikely, yet not unrealistic,
case where the bank's security gets compromised. This naturally raises the
question of whether disclosing secrets is fundamentally necessary for
identifying oneself, or more generally for proving a statement to be correct.
Developments in computer science provide an elegant solution via the concept of
zero-knowledge proofs: a prover can convince a verifier of the validity of a
certain statement without facilitating the elaboration of a proof at all. In
this work, we report the experimental realisation of such a zero-knowledge
protocol involving two separated verifier-prover pairs. Security is enforced
via the physical principle of special relativity, and no computational
assumption (such as the existence of one-way functions) is required. Our
implementation exclusively relies on off-the-shelf equipment and works at both
short (60 m) and long distances (400 m) in about one second. This demonstrates
the practical potential of multi-prover zero-knowledge protocols, promising for
identification tasks and blockchain-based applications such as cryptocurrencies
or smart contracts.
