Driven by exciting new research and applications, top-down and bottom-up fabrication techniques are producing ever more intricate, reproducible, plasmonic nano-architectures with gaps and junctions approaching the single nanometre and atomic scales. Such atomic-sized features promote the intersection of physics, chemistry and biology in plasmonics. Consequently, understanding light-matter interactions in such closely spaced, electromagnetically coupled, metallic nanosystems is of vital importance to a tremendous variety of current and future nanophotonic technologies. This thesis describes the first dynamically controlled, optically broadband, experimental investigations of light-driven plasmonic coupling between two metal nanostructures with sub-nanometre separation. A new experimental apparatus and nanosystem alignment technique was developed to enable the required sub-nanometre inter-nanoparticle geometry to be created and probed. Two conducting atomic force microscopy tips with nanoparticle functionalised apices are brought into nanoscale `tip-to-tip' axial alignment with dynamically-controlled spacing and ultra-wide optical access. Resonant electrical parametric mixing, created by oscillating the electromechanically coupled tips, is utilised to extract an electronic signal due to nanoscale changes in inter-tip position. Experimental results match theory confirming the viability of the technique. By functionalising the tip apices, this unique multi-functional observation platform allows the plasmonic response of nanoparticle dimers with sub-nanometre separations to be characterised. By simultaneously capturing both the electrical and optical properties of tip-mounted gold nanoparticles with controllable sub-nanometre separation, the first evidence for the quantum regime of optically driven tunnelling plasmonics is revealed in unprecedented detail. It is demonstrated that quantum mechanical effects are critically important at approximately the 0.3 nm scale where spatially non-local tunnelling plasmonics controls the optical response. All observed phenomena are in good agreement with a recently developed quantum-corrected model of plasmonic systems. The findings imply that tunnelling establishes a quantum limit for plasmonic field enhancement and confinement. Additionally, the work suggests the highly enhanced local density of photonic states in nanoscale cavities could enable coherent plasmon-exciton coupling. This thesis prompts new experimental and theoretical investigations into quantum-domain plasmonic systems, and impacts the future of nanoplasmonic device engineering, nanoscale photochemistry and plasmon-mediated electron tunnelling.