Invitation to a talk: Nikola Paunkovic, Friday Nov. 10th 2017, 14:30h at the IQOQI Seminar Room

We present a fair and optimistic quantum contract signing protocol between two clients that requires no communication with the third trusted party during the 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 scales as N^{−1/2} , where N is the number of messages exchanged between the clients. Our protocol is not based on the exchange of signed messages: its fairness is based on the laws of quantum mechanics. Thus, it is abuse-free, and the clients do not have to generate new keys for each message during the Exchange phase. We discuss a real-life scenario when the measurement errors and qubit state corruption due to noisy channels and imperfect quantum memories occur and argue that for real, good enough measurement apparatus, transmission channels and quantum memories, our protocol would still be fair. We present an alternative protocol, based on entangled pairs, that does not require stable quantum memories, and is therefore possible to implement with the current technology. Moreover, it does not reveal the contract to Trent, and it allows for a single agent to obtain a signed certificate (unlike the original protocol, in which both clients were required to be present in order to obtain signed certificates).