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Nanotechnology

Broadband wavelength-flattened directional coupler on silicon nitride for quantum photonic integrated circuits

Mauricio Gómez-Robles, Ferney Castro-Simanca, Essowéréo Tchao, Francisco A. Domínguez-Serna, Wencel De La Cruz Hernández, Karina Garay-Palmett, Rafael Salas‐Montiel

Published Jun 10, 2026• 0 citations
Publisher

Abstract

Integrated silicon nitride photonics has emerged as a key enabling technology for quantum technologies. Among the essential devices, beam splitters in the form of integrated directional couplers are widely used in circuits such as Hong-Ou-Mandel (HOM) interferometry. However, the wavelength-dependent nature of these couplers limits their use in experiments involving broadband photons. In this work, we experimentally demonstrate a wavelength-flattened directional coupler on the silicon nitride platform, exhibiting a nearly wavelength-flattened splitting ratio over the measured telecom wavelength range. This integrated photonic circuit is compatible with broadband quantum photonic circuits, including HOM interferometry, and supports the development of integrated single- and photon-pair sources.

DOI
10.1088/1361-6528/ae7b3c

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Integrated silicon nitride photonics has emerged as a key enabling technology for quantum technologies. Among the essential devices, beam splitters in the form of integrated directional couplers are widely used in circuits such as Hong-Ou-Mandel (HOM) interferometry. However, the wavelength-dependent nature of these couplers limits their use in experiments involving broadband photons. In this work, we experimentally demonstrate a wavelength-flattened directional coupler on the silicon nitride platform, exhibiting a nearly wavelength-flattened splitting ratio over the measured telecom wavelength range. This integrated photonic circuit is compatible with broadband quantum photonic circuits, including HOM interferometry, and supports the development of integrated single- and photon-pair sources.
The generation of pure single-photon states and their characterization are fundamental in quantum information processing applications like quantum computing. In this work, we design integrated optical directional couplers on a silicon nitride platform for heralded single-photon characterization. We assume the generation of such photons in silicon nitride waveguides through the spontaneous four-wave mixing process. We show that manipulating the waveguide geometry and dielectric contrast makes it possible to obtain nearly wavelength-independent directional coupler designs that perform as 50:50 beamsplitters over a broad bandwidth around the central design wavelength. Assuming two independent but identical heralded single-photon sources, we compare the visibility obtained by simulating the Hong-Ou-Mandel interference for three different 50:50 beamsplitters: an ideal one (perfectly wavelength-independent), an engineered one (nearly wavelength-independent), and an unengineered one (significantly wavelength-dependent). The purity obtained with the engineered beamsplitter closely matches the purity obtained with the ideal one, for which the interference visibility is a direct measure of the quantum state purity. This finding implies that the proposed engineered beamsplitter is suitable for characterizing heralded single-photon states, provided its bandwidth is 23 nm.
We demonstrate integrated photonic circuits for quantum devices using sputtered polycrystalline aluminum nitride (AlN) on insulator. On-chip AlN waveguide directional couplers, which are one of the most important components in quantum photonics, are fabricated and show the output power splitting ratios from 50:50 to 99:1. Polarization beam splitters with an extinction ratio of more than 10 dB are also realized from the AlN directional couplers. Using the fabricated AlN waveguide beam splitters, we observe Hong–Ou–Mandel interference with a visibility of 91.7 % ±5.66 % .
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.
Multi-wavelength quantum light sources, especially at telecom band, are extremely desired in quantum information technology. Despite recent impressive advances, such a quantum light source with high quality remains challenging. Here we demonstrate a multi-wavelength quantum light source using a silicon nitride micro-ring with a free spectral range of 200 GHz. The generation of eight pairs of correlated photons is ensured in a wavelength range of 25.6 nm. With device optimization and noise-rejecting filters, our source enables the generation of heralded single-photons - at a rate of 62 kHz with gh(2)(0)=0.014±0.001g^{(2)}_{h}(0)=0.014\pm0.001gh(2)​(0)=0.014±0.001, and the generation of energy-time entangled photons - with a visibility of 99.39±0.45%99.39\pm 0.45\%99.39±0.45% in the Franson interferometer. These results, at room temperature and telecom wavelength, in a CMOS compatible platform, represent an important step towards integrated quantum light devices for the quantum networks.
Trapped ions are one of the most advanced platforms for quantum technologies, with applications ranging from quantum computing to precision timekeeping. A crucial step towards more compact and scalable systems involves integrating photonic integrated circuits (PICs) into surface ion traps to enable on-chip light delivery and optical addressing of individual ions. Currently, most implementations rely solely on transverse-electric (TE) mode grating couplers, where the emitted light is polarized in the plane of the chip. In this work, we design, fabricate and characterize key silicon nitride (Si3N4) PIC components required for scaling trapped-ion based quantum systems to multiple operating zones, including incoupling structures, splitters, and grating couplers that support both TE and transverse-magnetic (TM) modes with comparable optical losses. We benchmark the PIC at 760 nm, which is a typical wavelength for Yb+-applications. The fabricated grating couplers enable the outcoupling of collimated free-space beams for both polarizations, exhibiting distinct emission angles. This dual-polarization capability gives more flexibility in polarization control and expands the accessible optical design space for trapped-ion quantum technologies.