A New Protocol for Loss-Tolerant Quantum Coin Tossing

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Quantum coin tossing is a cryptographic task in which two parties, Alice and Bob, wish to generate a shared random bit but do not necessarily trust each other. This task is completely impossible to realize with classical asynchronous communication but becomes at least partially feasible when quantum communication is also available. The best quantum protocol known so far, due to Ambainis, uses qutrits and is near optimal in the sense that either party can bias the outcome with at most a 75% probability of success. However, when the transmission of the link is below 50%, Ambainis'  protocol can be easily broken by a cheating Bob. This problem arises whenever there exists a conclusive measurement allowing Bob to obtain with certainty, although with a probability less than one, relevant information about the state sent by Alice. In this talk, we will present a new protocol for quantum coin tossing that does not suffer from this weakness and, as a consequence, is loss-tolerant. We discuss possible attacks and argue that the protocol is secure. Technologically, the implementation of this protocol is no more difficult than implementing entangled quantum key distribution with qubits. This is joint work with Guido Berlin, Gilles Brassard and Nicolas Godbout.