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Quantum contract signing

We present a probabilistic quantum contract signing protocol between two clients that requires no communication with the third trusted party during the commitment (i.e. signature exchange) phase. We discuss its fairness and show that it is possible to design such a protocol for which the probability of a dishonest client to cheat becomes negligible, and scales as N−1/2, where N is the size of the signature, in bits. This way, our protocol over performs the classical probabilistic protocol by Ben-Or et. al. [4], for which the probability to cheat can be as high as 1/4. We discuss the real-life scenario when the measurement errors and qubit state corruption due to noisy channels occur and argue that for real, good enough measurement apparatus and transmission channels, our protocol would still be fair. Our protocol could be implemented by today’s technology, as it requires in essence the same type of apparatus as the one needed for BB84 cryptography protocol. Finally, we show that it is possible to generalize our protocol to an arbitrary number of clients.