  The physics of light-matter interactions is strongly constrained by both the
small value of the fine-structure constant and the small size of the atom.
Overcoming these limitations is a long-standing challenge. Recent theoretical
and experimental breakthroughs have shown that two dimensional systems, such as
graphene, can support strongly confined light in the form of plasmons. These 2D
systems have a unique ability to squeeze the wavelength of light by over two
orders of magnitude. Such high confinement requires a revisitation of the main
assumptions of light-matter interactions. In this letter, we provide a general
theory of light-matter interactions in 2D systems which support plasmons. This
theory reveals that conventionally forbidden light-matter interactions, such
as: high-order multipolar transitions, two-plasmon spontaneous emission, and
spin-flip transitions can occur on very short time-scales - comparable to those
of conventionally fast transitions. Our findings enable new platforms for
spectroscopy, sensing, broadband light generation, and a potential test-ground
for non-perturbative quantum electrodynamics.
