Phd in PHYSICS

Thesis title: Semiconductor quantum dots for photonic quantum repeaters

Current information exchange is based on optical fibers and satellite communication via free-space links, where security is provided by mathematical complexity. However, it could potentially be threatened by paradigm shifts in computing technology. Encryption techniques using quantum key distribution based on entangled photons would allow for theoretical full secure communication. The very same platform, entangled photons, can also be employed as a core element to establish multi-node secure communication---a concept known as quantum network. For these reasons, entangled photon sources might be the core of future quantum networks for secure communication. In this thesis, I study GaAs/AlGaAs quantum dots as entangled photon sources. After giving a general overview on the fundamentals of photonic quantum networks and GaAs droplet-etched quantum dots, I mainly focus on two aspects of the development of this technology. First, limits of the source performance as entangled photon sources and second, applications of entangled photons from quantum dots for secure communication. The prior includes degrading effects of entanglement in these quantum dots, especially based on multiphoton emission and optical Stark effect induced by the particular entangled-photon generation technique, resonant two-photon excitation. The experimental results demonstrate that multiphoton emission is negligible under practical conditions, which is supported by a probabilistic model. The finite excitation laser pulse duration in resonant two-photon excitation, on the other hand, induces an optical Stark effect. The measurements in this thesis support the theoretical predictions and an entanglement reduction by increasing excitation laser pulse length is observed experimentally. If some conditions are met, GaAs/AlGaAs quantum dots emit highly entangled photons, which are utilized in the second part of this thesis by applying them in entanglement-based quantum key distribution protocols. The demonstrations range from the first implementation of quantum dots as entangled photon sources for secure communication in fiber and free-space, to a continuous secret key exchange over three days. The second test case, in particular, tackles the challenges of real-life applications such as sunlight and mild rain. At the end, I provide a brief outlook on how to use entangled photons from GaAs/AlGaAs quantum dots to transfer information from one node of a network, namely a quantum repeater, to another by proposing an experiment called remote quantum teleportation.