  Entangled multiphoton states lie at the heart of quantum information,
computing, and communications. In recent years, topology has risen as a new
avenue to robustly transport quantum states in the presence of fabrication
defects, disorder and other noise sources. Whereas topological protection of
single photons and correlated photons has been recently demonstrated
experimentally, the observation of topologically protected entangled states has
thus far remained elusive. Here, we experimentally demonstrate the topological
protection of spatially-entangled biphoton states. We observe robustness in
crucial features of the topological biphoton correlation map in the presence of
deliberately introduced disorder in the silicon nanophotonic structure, in
contrast with the lack of robustness in nontopological structures. The
topological protection is shown to ensure the coherent propagation of the
entangled topological modes, which may lead to robust propagation of quantum
information in disordered systems.
