Site-controlled Quantum Dot Arrays Edge-Coupled to Silicon Nitride Waveguides Achieve 10-Dot Emission

Scaling up photonic integrated circuits promises revolutionary advances in computing and beyond, but creating reliable light sources has remained a significant challenge. John O’Hara, Nicola Maraviglia, and Mack Johnson, working at the Tyndall National Institute alongside Jesper Håkansson, Salvador Medina, and Gediminas Juska, now report a breakthrough in precisely positioning and connecting arrays of quantum dots to silicon nitride waveguides. Their method achieves deterministic coupling of single-photon emitters to integrated photonic circuits, paving the way for scalable and reproducible quantum photonic devices. By fabricating nanoscale pillars around each quantum dot, the team demonstrates high-yield coupling with efficiencies currently exceeding 80 percent, and verifies single-photon emission directly into the chip’s waveguide network, representing a crucial step towards practical quantum technologies.

This work addresses a key challenge in quantum photonics, the need for bright, reliable single-photon sources seamlessly integrated with low-loss waveguides. The team achieves deterministic placement of nanoscale structures via molecular beam epitaxy, enhancing light extraction efficiency and facilitating strong light-matter interaction within the silicon nitride circuit. The team successfully coupled single quantum dots to high-quality silicon nitride resonators, demonstrating strong light-matter interaction and confirming their potential for quantum photonic applications. This platform represents a significant advancement towards building scalable quantum photonic integrated circuits with deterministic single-photon sources, paving the way for more complex quantum systems.

Site-Controlled Quantum Dots for Scalable Photonics

This research focuses on building scalable quantum photonic circuits, moving beyond isolated quantum emitters to systems where many qubits can interact and be controlled on a chip. Key to this approach are site-controlled, self-assembled quantum dots that emit single photons, the fundamental building blocks of quantum information. Researchers emphasize deterministic placement of these dots, allowing for precise positioning on the chip. Silicon nitride serves as the primary waveguide material, chosen for its low optical loss, compatibility with standard fabrication processes, and ability to guide light at wavelengths used for long-distance communication.

Lithium niobate is also employed for electro-optic modulation, enabling dynamic control of quantum states. The team utilizes droplet epitaxy to create high-quality quantum dots and patterned substrates to guide their growth and placement, combining the strengths of different materials to create a more versatile and efficient system. Achieving scalability requires precise control over quantum dot placement, uniformity, and coupling to waveguides. Researchers are addressing this through deterministic placement techniques and optimizing quantum dot quality and uniformity, ensuring they emit single photons with high purity and narrow linewidths. Efficient light coupling, on-chip control, and material compatibility are also crucial areas of focus. This work has potential applications in secure communication, quantum computing, quantum sensing, and quantum networking, pushing the boundaries of quantum photonics towards practical devices.

Deterministic Coupling of Quantum Dots to Waveguides

This research demonstrates a significant advance in the scalability of photonic integrated circuits through the successful active alignment and edge-coupling of an array of ten site-controlled gallium arsenide quantum dots to ten silicon nitride waveguides at cryogenic temperatures. The team achieved this by fabricating nanopillars deterministically aligned around each quantum dot, resulting in a high-yield, regular array of single-photon sources. Verification of triggered single-photon emission into the silicon nitride chip confirms the functionality of this integrated system, paving the way for more complex quantum photonic circuits. The consistently reproducible signal collected from each coupled quantum dot, achieving 0.

17 relative to free-space collection, highlights the reliability of the coupling process. Simulations and experimental observations suggest that current coupling efficiencies are promising, demonstrating a significant level of integration. The low inhomogeneous broadening observed across the emitter array enabled significant spectral overlap between adjacent quantum dots, a key requirement for efficient multi-photon entanglement and complex quantum operations. Researchers acknowledge that the simplified geometric model used in simulations may not fully capture the complexities of the fabricated pillars, and that further refinement is needed to accurately predict coupling efficiencies. Future work will likely focus on optimizing the fabrication process and exploring alternative designs to further enhance coupling efficiency and reduce system limitations. This achievement represents a crucial step towards building large-scale, integrated quantum photonic systems with potential applications in secure communication, quantum computing, and advanced sensing.