  Confining particles to distances below their de Broglie wavelength
discretizes their motional state. This fundamental effect is observed in many
physical systems, ranging from electrons confined in atoms or quantum dots to
ultracold atoms trapped in optical tweezers. In solid-state photonics, a
long-standing goal has been to achieve fully tunable quantum confinement of
optically active electron-hole pairs known as excitons. To confine excitons,
existing approaches mainly rely on material modulation, which suffers from poor
control over the energy and position of trapping potentials. This has severely
impeded the engineering of large-scale quantum photonic systems. In this
doctoral thesis, we demonstrate electrically controlled quantum confinement of
neutral excitons in two-dimensional semiconductors. By combining gate-defined
in-plane electric fields with inherent interactions between excitons and free
charges in a lateral p-i-n junction, we achieve tunable exciton confinement
lengths reaching values below 10 nm. Quantization of excitonic motion manifests
in the measured optical response as a ladder of discrete voltage-dependent
states below the continuum. Moreover, we observe that our confining potentials
lead to a strong modification of the relative wave function of excitons. We
further highlight the versatility of our approach by extending our confinement
scheme to create quantum-dot-like zero-dimensional structures with a fully
tunable confinement length. Our technique provides an experimental route
towards achieving polariton blockade and creating scalable arrays of identical
single-photon sources, which has wide-ranging implications for realizing
strongly correlated photonic phases and on-chip optical quantum information
processors.
