  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.
