This functionality, either between two or more parties, is used in communication networks, for instance in online gaming and in randomized consensus protocols (due to its equivalence to the leader election functionality, a fundamental primitive in distributed computing) and is also an integral component for secure function evaluation 3. One of the fundamental primitives in this setting is coin flipping, in which two spatially separated distrustful parties share a randomly generated bit, whose value must be unbiased 2.

However, many advanced cryptographic schemes belong to a model where the two parties do not trust each other and hence cannot collaborate. Indeed, information-theoretic security has been demonstrated for the task of distributing a secret key between two trusted and collaborating communicating parties using systems exploiting quantum effects 1. Quantum communications hold the promise of achieving a security level that is impossible to reach by classical means. Security is an imperative in all communication networks. Our results offer a useful toolbox for future secure quantum communications. Moreover, we provide combined quantum coin flipping protocols that are almost perfectly secure against bounded adversaries. The implementation is based on a practical plug and play system, developed by significantly enhancing a commercial quantum key distribution device. Here we experimentally implement a quantum coin flipping protocol that performs strictly better than classically possible over a distance suitable for communication over metropolitan area optical networks. Although it is known that quantum versions of these primitives can offer information-theoretic security advantages with respect to classical protocols, a demonstration of such an advantage in a practical communication scenario has remained elusive. These tasks are based on a handful of fundamental primitives, such as coin flipping, where two distrustful parties wish to agree on a randomly generated bit.

Performing complex cryptographic tasks will be an essential element in future quantum communication networks.

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