Periodically nonreciprocal transmission and entanglement in a non-Hermitian topological phase

Nonreciprocal devices are crucial components in optical information processing and chiral quantum networks. By investigating the topological property and the nonreciprocity in a one-dimensional, non-Hermitian microtoroid cavities array, we find that non-Hermitian topological phase transition periodically occurs, and the associated periodic edge-cavity nonreciprocity emerges and peaks at the single-cavity exceptional points (EPs) when the array of cavities is in the topologically nontrivial phase. This nonreciprocity is associated with the topologically protected chiral-mode excitation. It has practical applications in both classical and quantum nonreciprocal devices, such as optical isolators and nonreciprocal optomechanical entanglement devices. These nonreciprocities are topologically protected, thus are robust to the disorder of system parameters. Our work offers an alternative approach to generate periodic single-cavity nonreciprocity with non-Hermitian topological effect, and fundamentally advance the fields of topological physics and nonreciprocity optics.