We propose to deepen the understanding Quantum-Electro-Dynamics (QED) effects by investigating theoretically and experimentally various aspects of light matter interactions at the single-atom single-photon level. The work-horse of the experimental work will be a single Barium ion trapped and cooled in front of large aperture optical elements. We will demonstrate efficient coupling between a single trapped ion and its own resonance fluorescence using a very high numerical aperture spherical mirror (from NA=0.4 to NA=1), and investigate the underlying energy shifts, changes in spontaneous emission and atomic motional states. Using this set-up, we plan to investigate the discrepancy between experiment and theory of free space coupling, and anticipate that our experiment will provide solid grounds and motivation for that. We will also implement sideband cooling on the quadrupole transition and measure the vibrational ground state using Electromagnetically Induced Transparency (EIT), which has the potential to yield a quantum-noise-limited read out of atomic motion. Last, collective level shifts and super-radiant effects between two ions will be investigated in a linear Paul trap. We wish to solve issues that quantum technology addresses and deals with both timely subjects such as basic components for quantum networks and light-matter interfaces at a quantum level and fundamental questions of quantum optics.