Bosonic-Fermionic Transition in Ultrastrong Light-Matter Coupling

The interaction of light and matter in the ultrastrong coupling (USC) regime is at the forefront of modern research in quantum electrodynamics and condensed matter physics. The USC arises when the photon-excitation exchange rate becomes a significant fraction of their uncoupled mode frequencies. Theory predicts that as the number of participating electrons decreases, the Hamiltonian of the coupled system evolves from the bosonic Hopfield model to the fermionic Rabi model, which describes a two-level atom (a natural qubit) with implications for quantum information.¹ However, direct evidence is still missing. Here, we use THz time-domain spectroscopy (THz-TDS) to study ensembles of Si donors in GaAs coupled with plasmonic cavities. The 1s-2p transition of donors can behave like a well-defined two-level system, fitting better into the Rabi model. Besides, it lies in the THz range and is tunable with an external magnetic field, which allows mapping out the anti-crossing behavior near zero detuning. Moreover, Au nanoslots, as plasmonic cavities, effectively reduce the mode volume to boost the coupling strength and decrease the electron number. This project will uncover new physical insights into light-matter interactions and pave the way for quantum information processing.