Ultrastrong coupling is studied in a modulation-doped parabolic potential well coupled to an inductance-capacitance resonant circuit. In this system, in accordance to Kohn’s theorem, strong reduction of the energy level separation caused by the electron-electron interaction compensates the depolarization shift. As a result, a very large ratio of 27% of the Rabi frequency to the center resonance frequency as well as a polariton gap of width $2\pi \times 670\mathrm{GHz}$ are observed, suggesting parabolic quantum wells as the system of choice in order to explore the ultrastrong coupling regime.

• Received 31 January 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.106402