This thesis investigates quantum light–matter interactions in photonic-crystal cavities, focusing on two complementary projects. The first project develops a theoretical framework for Landau polaritons in terahertz cavities by computing ground-state current-current correlations in a two-dimensional electron gas under ultrastrong coupling (USC). We show that these correlations reveal an intrinsically squeezed ground state—an effect absent in conventional linear spectroscopy. The second project designs, fabricates, and characterizes a one-dimensional chiral photonic-crystal cavity that breaks time-reversal symmetry. The cavity consists of a silicon layer sandwiched between lightly doped indium antimonide (InSb) wafers, exploiting InSb’s low carrier mass and magnetoplasma nonreciprocity to support a single circularly polarized mode at 0.67 THz under a 0.3 T magnetic field, achieving a quality factor above 200. Systematic experiments—varying temperature, magnetic field, and polarization—together with simulations, confirm the cavity’s nonreciprocal behavior and robust mode confinement. Altogether, these studies deepen our understanding of USC-induced quantum effects in photonic cavities and introduce a versatile platform that enables precise manipulation of material properties by breaking key symmetries.