Coherent interaction between a gate-defined quantum dot and a terahertz split-ring resonator in the ultrastrong coupling regime

Hybrid excitations of light and matter in the ultrastrong coupling regime have been intensively investigated to explore novel material functions. However, realization of ultrastrong coupling in a-few-electron systems has been challenging because of the small electronic dipole moment. Here, we fabricate a gate-defined quantum dot (QD) in the vicinity of a gap of a terahertz (THz) split-ring resonator (SRR). By illuminating the system with external THz radiation, the QD shows anti-crossing behavior between the electronic states and the resonance mode of the SRR. Our result indicates that, owing to the field enhancement by the SRR, the system enters the ultrastrong coupling regime even when only a few electrons reside in the QD.