Collective, Coherent, and Ultrastrong Coupling of 2D Electrons with Terahertz Cavity Photons

TL;DR

This paper demonstrates collective ultrastrong coupling between 2D electrons and terahertz cavity photons, revealing new quantum phenomena and enabling advances in quantum technologies through high-Q cavity design and magnetic field control.

Contribution

It reports the first observation of collective ultrastrong light-matter coupling in a high-mobility 2D electron gas within a high-Q terahertz photonic-crystal cavity, highlighting a new regime of quantum interactions.

Findings

Achieved a cooperativity of ~360 indicating strong coupling.

Observed a sqrt(n_e) dependence of polariton splitting, confirming collective behavior.

Detected a narrow cyclotron resonance linewidth of 5.6 GHz at 2 K.

Abstract

Nonperturbative coupling of light with condensed matter in an optical cavity is expected to reveal a host of coherent many-body phenomena and states. In addition, strong coherent light-matter interaction in a solid-state environment is of great interest to emerging quantum-based technologies. However, creating a system that combines a long electronic coherence time, a large dipole moment, and a high cavity quality ($Q$) factor has been a challenging goal. Here, we report collective ultrastrong light-matter coupling in an ultrahigh-mobility two-dimensional electron gas in a high-$Q$ terahertz photonic-crystal cavity in a quantizing magnetic field, demonstrating a cooperativity of $\sim$360. The splitting of cyclotron resonance (CR) into the lower and upper polariton branches exhibited a $\sqrt{n_\mathrm{e}}$-dependence on the electron density ($n_\mathrm{e}$), a hallmark of collective vacuum Rabi splitting. Furthermore, a small but definite blue shift was observed for the polariton frequencies due to the normally negligible $A^2$ term in the light-matter interaction Hamiltonian. Finally, the high-$Q$ cavity suppressed the superradiant decay of coherent CR, which resulted in an unprecedentedly narrow intrinsic CR linewidth of 5.6 GHz at 2 K. These results open up a variety of new possibilities to combine the traditional disciplines of many-body condensed matter physics and cavity-based quantum optics.