Single-Photon Advantage in Quantum Cryptography Beyond QKD (2412.14993v1)

Abstract: In quantum cryptography, fundamental laws of quantum physics are exploited to enhance the security of cryptographic tasks. Quantum key distribution is by far the most studied protocol to date, enabling the establishment of a secret key between trusted parties. However, there exist many practical use-cases in communication networks, which also involve parties in distrustful settings. The most fundamental quantum cryptographic building block in such a distrustful setting is quantum coin flipping, which provides an advantage compared to its classical equivalent. So far, few experimental studies on quantum coin flipping have been reported, all of which used probabilistic quantum light sources facing fundamental limitations. Here, we experimentally implement a quantum strong coin flipping protocol using single-photon states and demonstrate an advantage compared to both classical realizations and implementations using faint laser pulses. We achieve this by employing a state-of-the-art deterministic single-photon source based on the Purcell-enhanced emission of a semiconductor quantum dot in combination with fast polarization-state encoding enabling a quantum bit error ratio below 3%, required for the successful execution of the protocol. The reduced multi-photon emission yields a smaller bias of the coin flipping protocol compared to an attenuated laser implementation, both in simulations and in the experiment. By demonstrating a single-photon quantum advantage in a cryptographic primitive beyond QKD, our work represents a major advance towards the implementation of complex cryptographic tasks in a future quantum internet.