In this chapter, we discuss cyclocarbonsCyclocarbons: ultimate quantum rings formed by a closed chain of carbon atoms. An analytic model for an ideal polyyne ring predicts tunable terahertzTerahertz (THz) transitions induced by an experimentally attainable external electric field in the plane of the ring. The absence of inversion symmetry in a polyyne ring formed by an odd number of dimers makes its optoelectronic properties differ drastically from an even-dimered ring. We show that adding an additional dimer is equivalent to piercing the ring with a huge magnetic field. We also propose a robust THz emission scheme using optical excitation, highlighting the potential for a new class of tunable THz emitters based on cyclocarbons.