Topologically protected energy-time entangled biphoton states in photonic crystals

The concepts of topological phases have been widely exploited in quantum optics in recent years. Here we demonstrate a topological insulator implementing topological protection of correlated biphoton states. A degenerate four-wave mixing (FWM) process of pseudospin states propagating along the topological interface is numerically simulated. Strikingly, the signal and idler photons generated from the FWM process are clarified to be entangled between two degrees of freedom—the frequencies of photon pairs and their time of arrival. The topological edge states of the pump, signal, and idler are robust against the sharp bends and defects, revealing the topological protection of energy-time entangled biphoton states. These findings could pave the way for unprecedented topological quantum devices.