Vacuum Bloch-Siegert Shift in Cyclotron Resonance

In a two-dimensional electron gas (2DEG) inside a terahertz (THz) cavity in a perpendicular magnetic field, we observed a clear resonance-frequency shift originating from the counter-rotating coupling between the electrons' cyclotron resonance and THz cavity modes. This shift can be understood as a vacuum Bloch-Siegert shift, arising from the ultrastrong counter-rotating coupling between the cyclotron-orbiting 2DEG and the vacuum fluctuations of the THz cavity modes. While such a shift has been difficult to observe clearly and is usually neglected under the rotating-wave approximation, here an unambiguous observation was made possible by the broken time-reversal symmetry of the 2DEG in a magnetic field and the use of an ultrahigh-mobility 2DEG, a high-quality-factor cavity, and circularly polarized THz radiation.