Topological insulators are materials that have a gapped bulk energy spectrum but contain protected in-gap states appearing at their surface. These states exhibit remarkable properties such as unidirectional propagation and robustness to noise that offer an opportunity to improve the performance and scalability of quantum technologies. For quantum applications, it is essential that the topological states are indistinguishable. We report high-visibility quantum interference of single-photon topological states in an integrated photonic circuit. Two topological boundary states, initially at opposite edges of a coupled waveguide array, are brought into proximity, where they interfere and undergo a beamsplitter operation. We observe Hong-Ou-Mandel interference with 93.1 ± 2.8% visibility, a hallmark nonclassical effect that is at the heart of linear optics-based quantum computation. Our work shows that it is feasible to generate and control highly indistinguishable single-photon topological states, opening pathways to enhanced photonic quantum technology with topological properties, and to study quantum effects in topological materials.
Entanglement is a valuable resource in quantum information technologies. The practical implementation of entangled photon sources faces obstacles from imperfections and defects inherent in physical systems, resulting in a loss or degradation of entanglement. The topological photonic insulators, however, have emerged as promising candidates, demonstrating an exceptional capability to resist defect-induced scattering, thus enabling the development of robust entangled sources. Despite their inherent advantages, building programmable topologically protected entangled sources remains challenging due to complex device designs and weak material nonlinearity. Here, we present a development in entangled photon-pair generation achieved through a nonmagnetic and tunable anomalous Floquet insulator, utilizing an optical spontaneous four-wave-mixing process. We verify the nonclassicality and time-energy entanglement of the photons generated by our topological system. Our experiment demonstrates a substantial enhancement in nonclassical photon-pair generation compared to devices reliant only on topological edge states. Our results could lead to the development of resilient quantum sources with potential applications in quantum technology. Published by the American Physical Society 2024
Photonic quantum technologies enter a new phase when realized in photonic integrated circuits, leading to a great advance in practical applications. In the pursuit of high integration density and low circuit complexity, ultracompact devices delivered by topology optimization offer a promising solution to miniaturize these photonic systems even further. However, their potential for quantum experiments has not yet been fully explored despite the constant development. In this work, we demonstrate multiphoton quantum interference using a topology-optimized tritter with a size of 8.0
μm
× 4.5
μm. We characterize the tritter and reconstruct its transfer matrix by means of single- and two-photon statistics. We also perform heralded three-photon quantum interference with the tritter. The measured four-fold coincidence features a peak with visibility of (-47.9
± 8.6)%, which is in fair agreement with the prediction of -55.8% estimated from the reconstructed transfer matrix. Our work confirms successful multiphoton quantum interference at an ultracompact interferometer and demonstrates the possibility of utilizing topology-optimized multiport interferometers in various fields of quantum technologies.