Quantum state distribution over vast distances is essential for global-scale quantum networks and fundamental test of quantum physics at space scale. While satellite platforms have demonstrated thousand-kilometer entanglement distribution, quantum key distribution and quantum teleportation with ground, future constellations and deep-space missions demand photon sources that are robust, compact, and power-efficient. Integrated photonics offers a scalable solution, yet a critical spectral gap persists. Although telecom-band integrated photon-pair sources are well established, near-visible photons offer distinct advantages for satellite-to-ground links by mitigating diffraction loss and maximizing the collection efficiency of optical telescopes. Scalable integrated sources in this regime have remained elusive due to the fundamental challenge of achieving anomalous dispersion in materials transparent at visible wavelengths. Here we bridge this gap by demonstrating an integrated near-visible photon-pair source based on a wide-bandgap, ultralow-loss, silicon nitride (Si$_3$N$_4$) microresonator. By engineering the dispersion of higher-order waveguide modes, we overcome the intrinsic normal dispersion limit to achieve efficient phase matching. The device exhibits a spectral brightness of 4.87$\times$10$^7$ pairs/s/mW$^2$/GHz and a narrow photon linewidth of 357 MHz. We report high-purity heralded single-photon generation with a heralding rate up to 2.3 MHz and a second-order correlation function as low as 0.0041. Furthermore, we observe energy-time entanglement with 98.4% interference visibility, violating the CHSH limit even at flux exceeding 40.6 million pairs/s. Combined with the proven radiation hardness of Si$_3$N$_4$, this source constitutes a flight-ready hardware foundation for daylight quantum communications and protocols requiring on-orbit multiphoton interference.
Quantum state distribution over vast distances is essential for global-scale quantum networks and fundamental test of quantum physics at space scale. While satellite platforms have demonstrated thousand-kilometer entanglement distribution, quantum key distribution and quantum teleportation with ground, future constellations and deep-space missions demand photon sources that are robust, compact, and power-efficient. Integrated photonics offers a scalable solution, yet a critical spectral gap persists. Although telecom-band integrated photon-pair sources are well established, near-visible photons offer distinct advantages for satellite-to-ground links by mitigating diffraction loss and maximizing the collection efficiency of optical telescopes. Scalable integrated sources in this regime have remained elusive due to the fundamental challenge of achieving anomalous dispersion in materials transparent at visible wavelengths. Here we bridge this gap by demonstrating an integrated near-visible photon-pair source based on a wide-bandgap, ultralow-loss, silicon nitride (Si3N4) microresonator. By engineering the dispersion of higher-order waveguide modes, we overcome the intrinsic normal dispersion limit to achieve efficient phase matching. The device exhibits a spectral brightness of 4.87×107 pairs/s/mW2/GHz and a narrow photon linewidth of 357 MHz. We report high-purity heralded single-photon generation with a heralding rate up to 2.3 MHz and a second-order correlation function as low as 0.0041. Furthermore, we observe energy-time entanglement with 98.4% interference visibility, violating the CHSH limit even at flux exceeding 40.6 million pairs/s. Combined with the proven radiation hardness of Si3N4, this source constitutes a flight-ready hardware foundation for daylight quantum communications and protocols requiring on-orbit multiphoton interference.
Photon pair sources are fundamental building blocks for quantum entanglement and quantum communication. Recent studies in silicon photonics have documented promising characteristics for photon pair sources within the telecommunications band, including sub-milliwatt optical pump power, high spectral brightness, and high photon purity. However, most quantum systems suitable for local operation (e.g., storage/computation) support optical transitions in the visible or short nearinfrared bands. In comparison to telecommunications wavelengths, the significantly higher optical attenuation in silica at such wavelengths limits the length scale over which optical-fiber-based quantum communication between such local nodes can take place. One approach to connect such systems over fiber is through a photon pair source that can bridge the visible and telecom bands, but an appropriate source, which should produce narrow-band photon pairs with a high signal-to-noise ratio, has not yet been developed in an integrated platform. Here, we demonstrate a nanophotonic visible-telecom photon pair source for the first time, using high quality factor silicon nitride resonators to generate narrow-band photon pairs with unprecedented purity and brightness, with coincidence-toaccidental ratio (CAR) up to 3,780 ± 140 and detected photon-pair flux up to (18,400 ± 1,000) pairs/s. We further demonstrate visible-telecom time-energy entanglement and its distribution over a 20 km fiber, far exceeding the fiber length over which purely visible wavelength quantum light sources can be transmitted. Finally, we show how dispersion engineering of the microresonators enables the connections of different species of trapped atoms/ions, defect centers, and quantum dots to the telecommunications bands for future quantum communication systems.
Photon-pair sources are critical building blocks for photonic quantum systems. Leveraging Kerr nonlinearity and cavity-enhanced spontaneous four-wave mixing, chip-scale photon-pair sources can be created using microresonators built on photonic integrated circuit. For practical applications, a high microresonator quality factor Q is mandatory to magnify photon-pair sources' brightness and reduce their linewidth. The former is proportional to Q^{4}, while the latter is inversely proportional to Q. Here, we demonstrate an integrated, microresonator-based, narrowband photon-pair source. The integrated microresonator, made of silicon nitride and fabricated using a standard CMOS foundry process, features ultralow loss down to 0.03 dB/cm and intrinsic Q factor exceeding 10^{7}. The photon-pair source has brightness of 1.17×10^{9} Hz/mW^{2}/GHz and linewidth of 25.9 MHz, both of which are record values for silicon-photonics-based quantum light source. It further enables a heralded single-photon source with heralded second-order correlation g_{h}^{(2)}(0)=0.0037(5), as well as an energy-time entanglement source with a raw visibility of 0.973(9). Our work evidences the global potential of ultralow-loss integrated photonics to create novel quantum light sources and circuits, catalyzing efficient, compact, and robust interfaces to quantum communication and networks.
We propose silicon nitride (Si3N4) nanowaveguide-based CMOS-compatible integrated sources of spectrally pure visible-telecom photon pairs. Our designs are intrinsically devoid of spectral correlations, eradicating the need for narrow-band filtering that reduces the source brightness. Further, the signal photons lie in the visible--near-infrared (vis-NIR) band, which can provide increased heralding efficiency of the idler photon in the telecom band using high-efficiency single-photon detectors. For example, a rectangular-cross-section silica-clad Si3N4 nanowaveguide with dimensions $680\text{~nm}\ifmmode\times\else\texttimes\fi{}840\text{~nm}$ and length 7 cm provides a heralded single-photon purity of approximately $91\mathrm{%}$ for a pump wavelength of 956.8 nm and a pump bandwidth of 1200 GHz, with the signal and idler lying in the visible and telecom bands at approximately 701 nm and approximately 1505 nm, respectively, demonstrating a peak spectral brightness of approximately $1.2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ photon pairs/s/nm/mW2 for a linear propagation loss of 0.5 dB/cm. The effect of propagation losses on the purity is also studied for such source designs. We also propose some designs capable of generating high-purity photon pairs to address various specific vis-NIR quantum memory transitions with telecom-band interfacing for long-distance hybrid quantum networks. Such compact on-chip sources could have potential applications in various quantum technologies, such as sensing, simulation, computing, communication, and networking.
Correlated photon-pair sources are key components for quantum computing, networking, and sensing applications. Integrated photonics has enabled chip-scale sources using nonlinear processes, producing high-rate entanglement with sub-100 microwatt power at telecom wavelengths. Many quantum systems operate in the visible or near-infrared ranges, necessitating broadband visible-telecom entangled-pair sources for connecting remote systems via entanglement swapping and teleportation. This study evaluates broadband entanglement generation through spontaneous four-wave mixing in various nonlinear integrated photonic materials, including silicon nitride, lithium niobate, aluminum gallium arsenide, indium gallium phosphide, and gallium nitride. We demonstrate how geometric dispersion engineering facilitates phase-matching for each platform and reveals unexpected results, such as robust designs to fabrication variations and a Type-1 cross-polarized phase-matching condition for III-V materials that expands the operational bandwidth. With experimentally attainable parameters, integrated photonic microresonators with optimized designs can achieve pair generation rates greater than ~1 THz/mW2.
Correlated photon-pair sources are key components for quantum computing, networking, synchronization, and sensing applications. Integrated photonics has enabled chip-scale sources using nonlinear processes, producing high-rate time-energy and polarization entanglement at telecom wavelengths with sub-100 microwatt pump power. Many quantum systems operate in the visible or near-infrared ranges, necessitating visible-telecom entangled-pair sources for connecting remote systems via entanglement swapping and teleportation. This study evaluates biphoton pair generation and time-energy entanglement through spontaneous four-wave mixing in various nonlinear integrated photonic materials, including silicon nitride, lithium niobate, aluminum gallium arsenide, indium gallium phosphide, and gallium nitride. We demonstrate how geometric dispersion engineering facilitates phase-matching for each platform and reveals unexpected results, such as robust designs to fabrication variations and a Type-1 cross-polarized phase-matching condition for III-V materials that expands the operational wavelength range.
Quantum networking enables the distribution of quantum information across distant nodes, unlocking capabilities such as secure communication, distributed quantum computing, enhanced metrology, and tests of fundamental physics. Photons serve as the natural carriers of quantum information over long distances due to their low loss in optical fibers and robustness to environmental decoherence. However, the practical realization of quantum networks remains limited by the lack of photon sources that are simultaneously compatible with diverse quantum systems, stable over long timescales, and suitable for deployment in real-world fiber infrastructure. This dissertation addresses these challenges through the co-development of photon-pair sources and a field-deployed quantum networking testbed. We present two complementary chip-scale sources of correlated photon pairs tailored for distinct bottlenecks in quantum networking. The first is a hybrid visible–telecom photon-pair source designed to address a fundamental spectral mismatch between quantum nodes and communication channels. While most quantum memories and processors operate at visible to near- nfrared wavelengths, optical fibers exhibit minimal loss in the telecommunications band. By integrating periodically poled nonlinear waveguides with photonic integrated circuits, we demonstrate a platform capable of directly generating visible–telecom photon pairs, enabling efficient interfacing between heterogeneous quantum systems and existing fiber networks. The second source is a fully packaged, broadband telecom photon-pair source based on silicon waveguide spirals, designed for stable, long-duration operation. This stability enables extended measurements necessary to overcome channel losses and environmental drifts in deployed fiber networks. These sources are evaluated within RoQNET (Rochester Quantum Network), a ∼20 km fiber link between the Rochester Institute of Technology and the University of Rochester. Using this platform, we demonstrate the transmission and on-chip modulation of heralded telecom single photons, verify their nonclassical statistics, and characterize long-term channel behavior. Additionally, we explore programmable time-bin entanglement distribution using spectral waveshapers, providing a flexible approach to quantum communication. Together, this work establishes a co-designed source–network framework for scalable, heterogeneous quantum networking.