  We have investigated light-matter hybrid excitations in a quantum dot
(QD)-terahertz (THz) optical resonator coupled system. We fabricate a
gate-defined QD in the vicinity of a THz split-ring resonator (SRR) by using a
AlGaAs/GaAs two-dimensional electron system (2DES). By illuminating the system
with THz radiation, the QD shows a current change whose spectrum exhibits
coherent coupling between the electrons in the QD and the SRR as well as
coupling between the 2DES and the SRR. The latter coupling enters the
ultrastrong coupling regime and the coupling between the QD and the SRR is also
very close to the ultrastrong coupling regime, despite the fact that only a few
electrons reside in the QD.
