Experimental cheat-sensitive quantum weak coin flipping

• Published in: Nature communications Vol. 14; no. 1; p. 1855
• Main Authors: Neves, Simon, Yacoub, Verena, Chabaud, Ulysse, Bozzio, Mathieu, Kerenidis, Iordanis, Diamanti, Eleni
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 03.04.2023; Nature Publishing Group; Nature Portfolio
• Subjects: 639/766/400/482; 639/766/483/481; Beam splitters; Benchmarks; Cheating; Communication; Communication networks; Communications networks; Cryptography; Humanities and Social Sciences; Information theory; multidisciplinary; Optical fibers; Optical switching; Photons; Physics; Probability; Protocol; Sanctions; Science; Science (multidisciplinary); Security; Task complexity
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-023-37566-x

As in modern communication networks, the security of quantum networks will rely on complex cryptographic tasks that are based on a handful of fundamental primitives. Weak coin flipping (WCF) is a significant such primitive which allows two mistrustful parties to agree on a random bit while they favor opposite outcomes. Remarkably, perfect information-theoretic security can be achieved in principle for quantum WCF. Here, we overcome conceptual and practical issues that have prevented the experimental demonstration of this primitive to date, and demonstrate how quantum resources can provide cheat sensitivity, whereby each party can detect a cheating opponent, and an honest party is never sanctioned. Such a property is not known to be classically achievable with information-theoretic security. Our experiment implements a refined, loss-tolerant version of a recently proposed theoretical protocol and exploits heralded single photons generated by spontaneous parametric down conversion, a carefully optimized linear optical interferometer including beam splitters with variable reflectivities and a fast optical switch for the verification step. High values of our protocol benchmarks are maintained for attenuation corresponding to several kilometers of telecom optical fiber. Quantum-enhanced versions of weak coin flipping (a cryptographic primitive where two mistrustful parties agree on a random bit while favouring opposite outcomes) have been proposed in the past but never realised. Here, the authors fill this gap by improving on a previous proposal and implementing it with single photons in a fibre-based setup.