Here we introduce the four core results published in the first reporting period. These results not only push the boundaries of fundamental research but also lay a solid foundation for semiconductor based quantum communications.
1. Revealing the 3D Structure of Quantum Dots: First, we made a breakthrough in studying the GaAs/AlGaAs quantum dots. Quantum dots are like tiny artificial atoms that can emit single photons. But to make sure they emit light efficiently and stably, we first need to really understand what they look like. Using advanced tools like high-resolution scanning transmission electron microscopy, selective chemical etching, and atomic force microscopy, we gave these quantum dots a "full-body scan". We found that these quantum dots, formed through a method called droplet etching and nanohole infilling, are far from simple spheres. By mapping out their 3D structure in detail, we now have a solid base for improving the growth process, allowing quantum dots to emit cleaner and more useful light. In short, this work is like drawing a detailed map of a tiny quantum dot, thus helping us build better quantum light sources.
2. Designing an Efficient and Tunable Light Extraction Device: While quantum dots can emit great photons, collecting these photons efficiently is a big challenge. A lot of the light gets lost right after it’s emitted. To solve this, we designed a brand-new photonic crystal grating structure. Through extensive computer simulations, our design showed excellent performance across the telecom C-band: it achieved a Purcell factor of 23 and a fiber coupling efficiency of over 90%. It means we can direct almost all of the photons from the quantum dots straight into optical fibers with low loss, and we can fine-tune their energy using electrical signals. This is a solid step toward building scalable, standardized quantum communication networks.
3. Connecting Quantum Dots and Diamond for Hybrid Quantum Networks: One big challenge in quantum communication and computing is how to connect different types of quantum systems effectively. The silicon-vacancy centers in diamonds are very stable quantum systems that can store quantum information for a long time, even at relatively high temperatures. But they don’t emit light very efficiently. So we had a new idea: why not use quantum dots to help? Through experiments, we successfully created GaAs/AlGaAs quantum dots that emit light at wavelength of the SiV centers of diamonds. What's more, these quantum dots can operate at 80 K (about -193°C), which is much "warmer" compared to the ultra-low temperatures (around 4K) usually needed. Even better, these quantum dots not only emitted very pure single photons but also generated entangled photon pairs. This breakthrough opens the door to connecting different quantum platforms seamlessly. It’s an important step toward building complex, city-scale or even country-scale quantum networks.
4. Achieving Intercity Quantum Key Distribution with Quantum Dot Single-Photon Sources: we performed the first intercity quantum key distribution (QKD) experiment that uses a deterministic quantum dot single-photon source. In simple terms, QKD is a technology for absolutely secure communication, powered by quantum physics. Most previous QKD systems used lasers and random filters to create photons, which had efficiency and security problems. A quantum dot single-photon source, however, emits exactly one photon at a time, solving these issues perfectly. In our experiment, we set up a 79 km link between Hannover and Braunschweig in Germany. The results were very promising: we achieved an extremely low error rate and a record-high secure key rate per pulse. This proves that with solid-state devices like quantum dots, we can now realistically support long-distance, practical quantum communication networks. In the future, this could even be extended to large-scale entanglement distribution, laying the foundation for a true quantum internet.
