Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols.The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching.Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam.The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW).Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source.This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.
Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols.The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching.Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam.The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW).Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source.This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.
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.
Broadband nonlinear optical devices play a critical role in both classical and quantum optics. Here, we design and fabricate a 6.82-mm-long step-chirped periodically poled lithium niobate~(CPPLN) waveguide on lithium niobate on insulator, which enables quasi-phase matching over a broad bandwidth for second-harmonic generation~(SHG) and spontaneous parametric down-conversion~(SPDC). The SHG achieves an average efficiency of 54.4\%/W/cm2 over the first-harmonic wavelength range of 1510~nm-1620~nm, paving the way for realizing SPDC across a wide range of pump wavelengths. For SPDC, by tuning the pump wavelength to 775~nm, 780~nm, and 785~nm, we achieve broadband photon-pair generation with a maximum full bandwidth and brightness up to 99~THz~(846~nm) and 20~GHz/mW/nm, respectively. Our findings provide an efficient and experiment-friendly approach for generating broadband photon pairs, which holds significant promise for advancing applications in quantum metrology.
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.
Broadband nonlinear optical devices play a critical role in both classical and quantum optics. Here, we design and fabricate a 6.82-mm-long step-chirped periodically poled lithium niobate (CPPLN) waveguide on lithium niobate on insulator, which enables quasi-phase matching over a broad bandwidth for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC). The SHG achieves an average efficiency of 91%/W/cm 2 over the first-harmonic wavelength range of 1510 nm–1620 nm, paving the way for realizing SPDC across a wide range of pump wavelengths. For SPDC, by tuning the pump wavelength to 775 nm, 780 nm, and 785 nm, we achieve broadband photon-pair generation with a maximum full bandwidth and brightness up to 99 THz (846 nm) and 21 MHz/mW/nm, respectively. Our findings provide an efficient and experiment-friendly approach for generating broadband photon pairs, which holds significant promise for advancing applications in quantum metrology.
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.
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.