We demonstrate terahertz chiral subwavelength cavities that break time-reversal symmetry by coupling the degenerate linearly polarized modes of two orthogonal sets of nanoantenna arrays using the inter-Landau-level transition of a two-dimensional (2D) electron gas in a perpendicular magnetic field, realizing normalized light-matter coupling rates up to ${\mathrm{\Omega }}_R/{\omega }_{\mathrm{cav}}=0.78$ with a dispersion that is modified by the parasitic capacitive coupling between the orthogonal antennas. The deep subwavelength confinement of the nanoantennas means that the ultrastrong-coupling regime can be reached even with a small number of carriers compared to Fabry-Pérot cavities, making it viable to be used with a variety of 2D materials. The nondegenerate circularly polarized ground state was only obtained after carefully optimizing the optical design to minimize the parasitic coupling to linearly polarized light.

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• Landau levels
• Nanoantennas
• Quantum states of light
• Cyclotron resonance
• Terahertz time-domain spectroscopy