Ultrastrong coupling between magnetoplasmons and cyclotron harmonics in terahertz resonator-quantum point contact integrated systems

In the ultrastrong light–matter coupling regime, vacuum field fluctuations play an essential role and can drive cavity-induced quantum phase transitions. To explore such phases, the light–matter coupling strength is a key parameter, and tunability of this coupling is highly desired. Although ultrastrong coupling has been demonstrated in various solid-state platforms, control of the coupling strength remains challenging. Here, we experimentally demonstrate ultrastrong coupling between localized plasmons and higher-harmonic cyclotron excitations in a terahertz (THz) resonator-quantum point contact (QPC) integrated system. We find that the coupling strength increases as the constriction of the QPC channel is narrowed and that the system enters the ultrastrong coupling regime. This tunability can be understood by the light-matter interaction with a spatially nonuniform plasmon field. Our highly controllable platform offers a promising route for exploring cavity-induced phases in condensed-matter systems. Ultrastrong light–matter coupling has been demonstrated in various solid-state platforms; however, controlling the coupling strength remains challenging. Here, the authors experimentally achieve electrostatic control of ultrastrong coupling between localized plasmons and higher-harmonic cyclotron excitations in a quantum point contact–terahertz resonator integrated system.