Multiphoton Experiments with Semiconductor Quantum Dots

Quantum physics has given us an understanding of the microscopic world and provided us with tools to quantitatively describe its puzzling phenomena. These tools have been exploited to engineer electronic devices and networks, which have led to radical changes in modern societies, sometimes called the quantum revolution. Based on the new ideas of quantum information processing, we are now on the verge of a second quantum revolu- tion, in which still-unexploited quantum phenomena could be turned into quantum comput- ers, capable of solving problems that are intractable with current technologies, and into quantum communication systems providing highest security. Among the different device implementations, photons the light quanta represent the natural choice for quantum communication and are also suitable for quantum computation purposes. One of the hur- dles towards these revolutionary applications has been the lack of light sources capable of emitting single and multiple photons on demand. Nanometre-sized structures of semicon- ducting materials, which are already the basis of classical computation and communication architectures, may provide the solution to this problem. Within this project, we aim to establish a world-leading photonic platform relying on an emerging class of semiconductor photon-sources combined with innovative photonic cir- cuits and use it to demonstrate multiphoton quantum protocols. To reach this goal, we will combine complementary expertise available at the Universities of Linz, Innsbruck, and Vi- enna. We will focus our efforts on so-called semiconductor quantum dots made of gallium ar- senide. When operated at cryogenic temperatures, such quantum dots have recently shown several appealing features, such as the capability of generating single and entan- gled photons at gigahertz rates, with their light color matched to the high-sensitivity range of silicon-based detectors. Substantial efforts are however necessary to increase the source brightness and the quality of the photons. If photons from multiple sources need to be combined they must be all identical. We will tackle these challenges by (i) integrating the quantum dots in photonic structures allowing efficient feeding of the emitted light into the photonic circuits, (ii) fine-tuning the color of the emitted photons through a patented technology, and (iii) exploring different quantum dot pumping schemes to increase the pu- rity of the emitted photons. In parallel to the progress in source performance, we will design applications with in- creasing complexity and implement them in high-performance photonic chips. These will include the generation of cluster states consisting of several entangled photons for secure quantum computing. Suitable tests will be designed to verify the generation of such states in the experiment and benchmark the overall performance. The combination of quantum light emitted by the quantum dots and novel integrated photonic circuits make this project unique. In the long term, we expect this approach to allow us exploring the ultimate limits of photonic-quantum-information processing.

Quantum physics provides a deep understanding of the microscopic world and the tools to describe its puzzling phenomena. Building on quantum information processing, we are on the verge of a "second quantum revolution," in which previously unexploited quantum effects could lead to quantum computers and quantum networks. Photons are particularly attractive for these applications, but until now there has been a lack of light sources that emit single and multiple photons on demand. Nanometre-scale semiconductor structures offer a solution. The goal of this project-a collaboration between the University of Linz, the University of Innsbruck, and the University of Vienna-was to build a world-class photonic platform based on semiconductor quantum dots that generates single and entangled photons on demand and uses them for increasingly complex applications. As planned, we integrated gallium-arsenide quantum dots into electrically and strain-tunable devices. We developed circular Bragg grating resonators (CBRs) for efficient coupling into free space and photonic chips, and realized diode-CBRs that combine high brightness, Purcell enhancement, Fourier-limited photons, and wavelength tunability. Within the project, we carried out one of the first entanglement-based QKD demonstrations with quantum-dot photons. Thanks to the brighter sources, quantum teleportation as well as entanglement swapping with independent quantum dots were demonstrated. In parallel, we developed robust excitation methods: adiabatic rapid passage with chirped laser pulses for reliable population inversion in multiple dots; the first experimental demonstration of SUPER excitation; and the preparation and use of the long-lived dark exciton as a photon source. On the theory side, we investigated entanglement and correlations in many-body systems as well as the indistinguishability of multiple bosons. We extended open-source simulation frameworks with new numerical methods for larger, more complex systems. Using these tools, we optimized laser pulses for fast, efficient generation of entangled photons via SUPER and predicted mirrorless lasing in linear arrays of quantum emitters with partial pumping-an advance toward nanoscale frequency references. With photons from quantum dots, we demonstrated two complementary routes to scaling photonic quantum processors: (1) the heralded generation of three-photon GHZ states-a key resource for cluster-state quantum computing-by demultiplexing a single-photon stream and processing it in an interferometric polarization network; (2) a time-bin encoding that exploits intrinsically sequential emission; using only a single spatial mode and a high-speed optical switch, we built a programmable multiphoton interference network with reduced hardware complexity while maintaining scalability. These advances bring practical, chip-scale quantum technologies significantly closer to application.