Collective non-perturbative coupling of 2D electrons with high-quality-factor terahertz cavity photons

The collective interaction of electrons with light in a high-quality-factor cavity is expected to reveal new quantum phenomena(1-7) and find applications in quantum-enabled technologies(8,9). However, combining a long electronic coherence time, a large dipole moment, and a high quality-factor has proved difficult(10-13). Here, we achieved these conditions simultaneously in a two-dimensional electron gas in a high-quality-factor terahertz cavity in a magnetic field. The vacuum Rabi splitting of cyclotron resonance exhibited a square-root dependence on the electron density, evidencing collective interaction. This splitting extended even where the detuning is larger than the resonance frequency. Furthermore, we observed a peak shift due to the normally negligible diamagnetic term in the Hamiltonian. Finally, the high-quality_factor cavity suppressed superradiant cyclotron resonance decay, revealing a narrow intrinsic linewidth of 5.6 GHz. High-quality-factor terahertz cavities will enable new experiments bridging the traditional disciplines of condensed-matter physics and cavity-based quantum optics.