The aim of this thesis is to explore nonlinear nanostructures – photonic systems with one or more dimension with nanoscale feature size – as new platforms for parametric photon-pair generation. This is motivated by the need for photon-pair sources in many technological applications of quantum optics, such as quantum communication, imaging and computing. The initial hypothesis is that nonlinear nanostructures are a promising platform to complement established photon-pair source schemes based on bulk crystals or photonic integrated circuits in terms of quantum state-engineering, footprint, functional density etc. However, nonlinear pair generation, e.g. via spontaneous parametric down-conversion (SPDC), is a new subfield in nanophotonics with a very limited number of studies existing. This constitutes the knowledge-gap addressed in this thesis. Therefore, the objective is to provide first experimental demonstrations of SPDC in subwavelength-scale and nonlinear nanostructures. We investigate the ability of such systems to control the properties of two-photon quantum states such as frequency and angular spectrum or polarization entanglement. To this end we analyze nanostructures with different degrees of complexity, starting with an unstructured thin crystal with one nanoscale dimension, leading up to a 3D nano-scale resonator with subwavelength-sized features in all dimensions. Furthermore, we test different types of nonlinear materials such as lithium niobate, group III-V semiconductors and emerging, highly nonlinear materials such as molybdenum disulfide. Additionally, we also advance the theoretical understanding by developing a comprehensive model of the quantum process of nonlinear parametric pair-generation in photonic nanostructures.
Maximilian Weißflog (Thu,) studied this question.