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APL Photonics

Versatile on-chip reconfigurable two-photon interference platforms using thin-film lithium niobate

Minho Choi, Changhyun Kim, Sunghyun Moon, Hyeon Hwang, Min‐Kyo Seo, Hyang‐Tag Lim, Haejun Chung, Hojoong Jung, Hyounghan Kwon

Published Jun 1, 2026• 0 citations

Abstract

Integrated quantum photonic technologies require scalable devices with high-visibility quantum interference. Recent studies have demonstrated that lithium niobate on insulator enables efficient nonlinear interactions, via quasi-phase matching in periodically poled lithium niobate (PPLN) waveguides. However, the on-chip control of two-photon interference utilizing path entanglement remains limited. In this study, we demonstrate on-chip generation of single-photon sources via quantum interference using path-entangled N00N states in two ways. Using the basic structure of a reverse Hong–Ou–Mandel (RHOM) configuration, we directly observe high on-chip visibility of quantum interference. We achieve a visibility of 97.7% ± 1.1% using a dual-PPLN waveguide as the basic RHOM configuration. To improve the compactness, efficiency, and stability of on-chip operation, we further incorporate a PPLN ring resonator with a Sagnac loop. This allows counter-propagating paths to share a single periodically poled waveguide, achieving a visibility of 84.4% ± 1.6%. These results demonstrate that lithium niobate on insulator (LNOI)-based quantum photonic systems provide a robust foundation for the development of future integrated quantum technologies.

DOI: 10.1063/5.0315916

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Interference between single photons is key for many quantum optics experiments and applications in quantum technologies, such as quantum communication or computation. It is advantageous to operate the systems at telecommunication wavelengths and to integrate the setups for these applications in order to improve stability, compactness and scalability. A new promising material platform for integrated quantum optics is lithium niobate on insulator (LNOI). Here, we realise Hong-Ou-Mandel (HOM) interference between telecom photons from an engineered parametric down-conversion source in an LNOI directional coupler. The coupler has been designed and fabricated in house and provides close to perfect balanced beam splitting. We obtain a raw HOM visibility of (93.5 ± 0.7) %, limited mainly by the source performance and in good agreement with off-chip measurements. This lays the foundation for more sophisticated quantum experiments in LNOI.

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Abstract Entanglement is central to quantum technologies such as cryptography, sensing, and computing. Photon pairs generated via nonlinear optical processes are excellent for preparing entangled states due to their long coherence times and compatibility with fiber optic networks. Steady progress in nanofabrication has positioned lithium niobate-on-insulator (LNOI) as a leading platform for monolithic integration of photon pair sources into optical circuits, leveraging its strong second-order nonlinearity. Here, we present a reconfigurable photonic integrated circuit on LNOI, which combines two on-chip photon pair sources with programmable interferometers, enabling the generation of entangled states. The photon pair sources achieve a source brightness of 26 MHz nm −1 mW −1 while maintaining a coincidence-to-accidental ratio above 100. We successfully interfere the two sources with 99.0 ± 0.7% visibility, demonstrating the indistinguishability required for producing entanglement on-chip. We show the preparation of any of the maximally entangled Bell states with fidelity above 90% verified by quantum state tomography. These results establish LNOI as a compelling, scalable platform to explore integrated quantum photonic technologies enabled by high-brightness sources of entangled quantum states.