  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 propagation direction, for correctly deducing the
offset between clocks -- a condition that could be manipulated with 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 successfully evading detection.
