Current Fluctuations and Quantum Squeezing in Landau Polaritons in the Ultrastrong Coupling Regime

Landau polaritons arise when the collective cyclotron resonance excitation of an electron gas strongly couples with cavity photons. When the coupling strength becomes a significant fraction of the bare cyclotron and cavity frequencies, the system enters the ultrastrong coupling (USC) regime, where the ground state is predicted to acquire non-classical properties. Typically, however, the linear optical response of Landau polaritons in the USC regime can be described well through classical Maxwell's equations, and the quantum nature of such a light–matter hybrid is yet to be convincingly proven. Here, we theoretically explore quantum fluctuation properties of Landau polaritons and possible quantum squeezing phenomena in their transport properties. We calculated the correlation functions of electric current and polarization in the ground state of a two-dimensional electron gas in a terahertz cavity in the USC regime and investigated the coupling-strength and magnetic-field dependences of the correlation functions. We found distinct modifications of the current fluctuation correlations, which might reflect the influence of two-mode squeezing that is expected to exist in the ground states of light–matter hybrids in the USC regime.