Asymmetric delay attack on an entanglement-based bidirectional clock synchronization protocol

We demonstrate an attack on a clock synchronization protocol that attempts to detect tampering of the synchronization channel using polarization-entangled photon pairs. The protocol relies on a symmetrical channel, where propagation delays do not depend on the propagation direction, for correctly deducing the offset between clocks—a condition that could be manipulated using optical circulators, which rely on static magnetic fields to break the reciprocity of propagating electromagnetic fields. Despite the polarization transformation induced within a set of circulators, our attack creates an error in time synchronization while evading detection.