Topological protection of two-photon quantum correlation on a photonic chip

The low-decoberence regime plays a key role in constructing multi-particle quantum systems and has therefore been constantly pursued in order to build quantum simulators and quantum computers in a scalable fashion. Quantum error correction and quantum topological computing have been proved to be able to protect quantumness but have not yet been experimentally realized. Recently, topological boundary states are found to be inherently stable and are capable of protecting physical fields from dissipation and disorder, which inspires the application of such topological protection on quantum correlation. Here we present an experimental demonstration of topological protection of two-photon quantum states against the decoherence in diffusion on a photonic chip. By analyzing the quantum correlation of photons out from the topologically nontrivial boundary state, we obtain a high cross-correlation and a strong violation of Cauchy-Schwarz inequality up to 30 standard deviations. We further prepare different quantum sources and experimentally confirm that the topological protection is robust to the wavelength difference as well as distinguishability of two photons. Our results, together with our integrated implementation, provide an alternative way of protecting quantumness and may inspire many more explorations in "quantum topological photonics", a crossover between topological photonics and quantum information. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement