Final Report Summary - PACART (Free space photon atom coupling - the art of focusing) The project PACART was devoted to enhancing the interaction of light and matter in free space by focusing light from full solid angle. The central experimental tool used in the project is a parabolic mirror with a depth much larger than its focal length. With such a mirror, a suitably shaped incident mode of the light field is converted into an electric-dipole wave, which is expected to maximize the coupling of light and single atoms or dipole-like quantum targets in general. In the experiments conducted in PACART, this coupling scheme has been applied for coupling light to single ions, for trapping a solid-state quantum-target, as well as for investigating nonlinear optics in a gaseous medium in an exciting and so-far unexplored regime.In the experiments with a single ion, considerable coupling efficiencies have been achieved as e.g. witnessed by the phase shift of a weak coherent state of light. Furthermore, several technologies for compensating the aberrations of deep parabolic mirrors were explored, leading the path to diffraction limited and therefore most efficient full solid angle (4π) focusing in free space.The optical trap hosting solid-state quantum targets, which is based on the same design principle as the coupling of light to a single ion, yields trapping performances much better than the ones obtained by standard focusing in free space. The successful verification of single-photon emission from the trapped nanoparticles envisions the realization of an efficient single-photon source build upon technologically relevant solid-state quantum systems.Along with these experiments on ultimate focusing, several experimental technologies applicable in – but not limited to – coupling single atoms and light in free space were developed. This includes the generation of spectrally shaped single photons by parametric down-conversion in a whispering-gallery-mode resonator and the demonstration of bright squeezed vacuum with tailored spectral properties.
