Landau Polaritons in Chiral Photonic Cavities

Vacuum fluctuations can modify the ground state of a light-matter coupled system, especially in the ultrastrong coupling regime (where coupling strength is a significant fraction of light/matter resonant frequencies). Here, we introduce a chiral cavity that confines a single circularly polarized mode, thereby realizing a time-reversal symmetry-breaking, single-handed vacuum. We study the Landau polariton system in the chiral cavity, and show that the collective cyclotron motion of electrons couples differently to each polarization, producing exactly two tunable polariton modes (unlike three in the achiral case). When the vacuum polarization co-rotates with the electrons, the polariton modes avoid each other; when they counter-rotate, the modes cross with vacuum-mediated energy shifts. The polariton modes in the crossing and anti-crossing cases have different symmetries set by the vacuum handedness, show enhanced Bloch–Siegert shift, and a squeezed ground state. With this work, we demonstrate that cavity-vacuum handedness controls internal resonances, opening new routes in terahertz technologies and fundamental studies of quantum-vacuum phenomena in condensed matter.