Collective magneto-polariton excitation in a terahertz photonic cavity

Collective excitations in solids offer new opportunities for quantum optical studies. Many-body interactions inherent to condensed matter systems can lead to novel phenomena that cannot be achieved in traditional atomic systems. Here, we report collective ultrastrong light-matter coupling in a two-dimensional electron gas in a high-$Q$ terahertz photonic-crystal cavity in a magnetic field. We directly observed time-domain vacuum Rabi oscillations, whose frequency was found to be proportional to the square root of $N$ (where $N$ is the carrier density), evidence for the {\em collective} nature of ultrastrong coupling. In addition, a small but definite blue shift due to the diamagnetic term in the Hamiltonian was observed for the polariton frequencies, which is another signature of ultrastrong light-matter coupling. Furthermore, the high-$Q$ cavity suppressed the superradiant decay of cyclotron resonance, which resulted in unprecedentedly narrow intrinsic cyclotron resonance linewidths ($\sim$5.6 GHz at 2 K). Our method is also applicable to many classes of strongly correlated systems with collective many-body excitations in the terahertz range, opening a door to the fascinating physics of terahertz many-body cavity QED.