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Integrated Optics: Devices, Materials, and Technologies

High-performance photon-pair source using a two-photon polymerized strip-loaded lithium niobate waveguide

Muhamed A. Sewidan, Alexandra Rittmeier, Laura Bollmers, Silia Babel, Elisavet Chatzizyrli, J. S. S. Durán Gómez, Laura Padberg, Christof Eigner, Christine Silberhorn, Douglas Bremner, Andreas Wienke, Dietmar Kracht, Moritz Hinkelmann, Michael Kues

Published Jan 24, 2025• 0 citations
Publisher

Abstract

Integrated photon-pair sources, providing scalability solutions, are crucial for large-scale realizations of quantum photonic technologies. Their traditional fabrication techniques, such as etching, are often complex, require long development cycles, and are resource inefficient in case of fabrication imperfections or failures. The two-photon polymerization (2PP) technique can offer a rapid, efficient, and reproducible alternative for the fabrication of integrated photonic devices. Here, we demonstrate an integrated photon-pair source based on a 2PP-defined strip-loaded waveguide on a periodically poled thin-film lithium niobate (TFLN) platform. The device generates photon pairs through type-II spontaneous parametric down-conversion (SPDC), employing first-order quasi-phase matching (QPM) with TM-polarized excitation light. The device achieves an on-chip pair generation rate of 4.357 MHz and a coincidence-to-accidental ratio (CAR) of ∼404. To prove the flexibility and versatility of the fabrication technique, the waveguides were erased from the LN substrate and reprinted, reaching comparable performance. This work demonstrates the potential of 2PP as a rapid production technique for high-performance integrated photon-pair sources, offering a practical solution to the limitations of traditional fabrication techniques and showcasing both reproducibility of the fabrication and the reusability of the substrate.

DOI
10.1117/12.3041187

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Integrated photon-pair sources, providing scalability solutions, are crucial for large-scale realizations of quantum photonic technologies. Their traditional fabrication techniques, such as etching, are often complex, require long development cycles, and are resource inefficient in case of fabrication imperfections or failures. The two-photon polymerization (2PP) technique can offer a rapid, efficient, and reproducible alternative for the fabrication of integrated photonic devices. Here, we demonstrate an integrated photon-pair source based on a 2PP-defined strip-loaded waveguide on a periodically poled thin-film lithium niobate (TFLN) platform. The device generates photon pairs through type-II spontaneous parametric down-conversion (SPDC), employing first-order quasi-phase matching (QPM) with TM-polarized excitation light. The device achieves an on-chip pair generation rate of 4.357 MHz and a coincidence-to-accidental ratio (CAR) of ∼404. To prove the flexibility and versatility of the fabrication technique, the waveguides were erased from the LN substrate and reprinted, reaching comparable performance. This work demonstrates the potential of 2PP as a rapid production technique for high-performance integrated photon-pair sources, offering a practical solution to the limitations of traditional fabrication techniques and showcasing both reproducibility of the fabrication and the reusability of the substrate.
Abstract Integrated photon-pair sources are crucial for scalable photonic quantum systems. Thin-film lithium niobate is a promising platform for on-chip photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the device implementation faces practical challenges. Periodically poled lithium niobate (PPLN), despite enabling flexible quasi-phase matching, suffers from poor fabrication reliability and device repeatability, while conventional modal phase matching (MPM) methods yield limited efficiencies due to inadequate mode overlaps. Here, we introduce a layer-poled lithium niobate (LPLN) nanophotonic waveguide for efficient photon-pair generation. It leverages layer-wise polarity inversion through electrical poling to break spatial symmetry and significantly enhance nonlinear interactions for MPM, achieving a notable normalized second-harmonic generation (SHG) conversion efficiency of 4615% W-1 cm-2. Through a cascaded SHG and SPDC process, we demonstrate photon-pair generation with a normalized brightness of 3.1×106 Hz nm-1 mW-2 in a 3.3 mm long LPLN waveguide, surpassing existing on-chip sources under similar operating configurations. Crucially, our LPLN waveguides offer enhanced fabrication reliability and reduced sensitivity to geometric variations and temperature fluctuations compared to PPLN devices. We expect LPLN to become a promising solution for on-chip nonlinear wavelength conversion and non-classical light generation, with immediate applications in quantum communication, networking, and on-chip photonic quantum information processing.
Thin-film lithium niobate (TFLN) waveguides have emerged as a pivotal platform for on-chip spontaneous parametric down-conversion (SPDC), serving as a crucible for the generation of entangled photon pairs. The periodic poling of TFLN, while capable of generating high-efficiency SPDC, demands intricate fabrication processes that can be onerous in terms of scalability and manufacturability. In this work, we introduce a novel approach to the generation of entangled photon pairs via SPDC within TFLN waveguides, harnessing the principles of modal phase-matching (MPM). To address the challenge of efficiently exciting pump light typically in a higher-order mode, we have engineered a mode converter that couples two asymmetrically dimensioned waveguides. This converter adeptly transforms the fundamental mode into a higher-order mode, demonstrating a conversion loss of 1.55 dB at 785 nm with a 3 dB bandwidth exceeding 30 nm. Subsequently, we have showcased the device's capabilities by characterizing the pair generation rate (PGR), coincidences-to-accidentals ratio (CAR), and spectral profile of the entangled photon source. Our findings present a simplified and versatile method for the on-chip generation of entangled photon sources, which may pave the way for the application in the realms of quantum information processing and communication technologies.
Spontaneous parametric down-conversion (SPDC) has become a key method for generating entangled photon pairs. Periodically poled thin-film lithium niobate (TFLN) waveguides induce strong SPDC but require complex fabrication processes. In this work, we experimentally demonstrate efficient SPDC and second harmonic generation using modal phase matching methods. This is achieved with inverse-designed optical mode converters and low-loss optical waveguides in a single nanofabrication process. Inverse design methods provide enhanced functionalities and compact footprints for the converter. Despite the extensive achievements in inverse-designed photonic integrated circuits, the potential of inverse-designed TFLN quantum photonic devices has been seldom explored. The device shows an on-chip conversion efficiency of 3.95% W−1 cm−2 in second harmonic generation measurements and a coincidence count rate up to 21.2 kHz in SPDC experiments. This work highlights the potential of the inverse-designed TFLN photonic devices and paves the way for their applications in on-chip nonlinear or quantum optics.
Spontaneous parametric down conversion in nonlinear material is widely exploited to generate entangled photon pairs in quantum optics experiments and applications, including quantum computing and communication. Periodically poled thin film lithium niobate (PPTFLN) has emerged as a promising platform for efficient entangled photon pair generation, offering enhanced nonlinear interaction through quasi-phase matching (QPM) and tight confinement of light. However, achieving optimal performance requires careful control of the QPM condition since the waveguide in TFLN is highly dispersive to changes in the geometric parameter. In this study, we fabricate PPTFLN rib waveguides to generate entangled photon pairs at telecommunication wavelengths, varying geometric parameters. QPM condition is confirmed with the second harmonic generation experiments and Pair generation rate and coincidence-to-accidental count ratio are also estimated by temporal coincidence measurement. Digital etching process is introduced to control the QPM condition, resulting in incremental peak wavelength shift by discrete etching step. This is expected to contribute to synchronizing wavelength of quantum nodes.
A thin-film periodically poled lithium niobate waveguide was designed and fabricated which generates entangled photon pairs at telecommunications wavelengths with high coincidences-to-accidentals counts ratio CAR>67000, two-photon interference visibility V>99%, and heralded single-photon autocorrelation g_{H}^{(2)}(0)<0.025. Nondestructive in situ diagnostics were used to determine the poling quality in 3D. Megahertz rates of photon pairs were generated by less than a milliwatt of pump power, simplifying the pump requirements and dissipation compared to traditional spontaneous parametric down-conversion lithium niobate devices.
Chip-integrated photon-pair sources based on spontaneous parametric down-conversion (SPDC) have emerged as a promising solution for scalable quantum light generation. Thin-film lithium tantalate (TFLT) is a compelling χ(2)χ^{(2)}χ(2) platform, combining strong nonlinearity with a high optical-damage threshold, weak photorefractive response, and ferroelectricity that enables quasi-phase matching. However, SPDC-based photon-pair generation on TFLT has not yet been demonstrated. Here, we combine high-quality periodic poling with low-loss nanophotonic waveguides to realize photon-pair sources on TFLT in both traveling-wave and resonant configurations. In periodically poled straight waveguides, we achieve broadband photon-pair generation with high efficiency (2.1 GHz mW−12.1~\mathrm{GHz}~\mathrm{mW}^{-1}2.1 GHz mW−1) and coincidence-to-accidental ratio (up to 3.8×1053.8\times10^{5}3.8×105). We further confirm high-purity single-photon operation via heralded second-order correlation (gH(2)(0)=0.0018±0.0002g^{(2)}_\mathrm{H}(0) = 0.0018 \pm 0.0002gH(2)​(0)=0.0018±0.0002) and high-fidelity time-energy entanglement through Franson interference (visibility of 98.9±0.5%98.9 \pm 0.5\%98.9±0.5%). In periodically poled racetrack resonators, we map out a broad quantum frequency comb spanning the telecom C- and L-bands. By isolating individual frequency-correlated pairs, we measure a high spectral brightness of 11 GHz mW−1 GHz−111~\mathrm{GHz}~\mathrm{mW}^{-1}~\mathrm{GHz}^{-1}11 GHz mW−1 GHz−1. These results are competitive with the state of the art across χ(2)χ^{(2)}χ(2) integrated platforms, positioning TFLT as a strong contender for integrated quantum light sources, with applications in wavelength-multiplexed quantum communications and photonic quantum information processing.