Gauge ambiguities in QED: Jaynes-Cummings physics remains valid in the ultrastrong-coupling regime.
Adam Stokes, Ahsan Nazir · arXiv (Cornell University) · 2018
Ultrastrong-coupling between an effective two-level system and radiation offers immense potential for advancing both fundamental and applied quantum electrodynamics (QED). Such regimes are identified by the break down of the rotating-wave approximation, which applied to the quantum Rabi model yields the apparently less fundamental Jaynes-Cummings model. However, the implications of gauge-freedom for ultrastrong-coupling QED are yet to be fully recognised. Here we show that when truncating the material system to only two levels, each gauge gives a different effective description whose predictions can vary significantly for ultrastrong-coupling. Rabi models are obtained through specific gauge choices, but so too is a Jaynes-Cummings model without needing the rotating-wave approximation. Analysing a circuit QED setup, we find that this Jaynes-Cummings model is often more accurate than the Rabi model. Thus, Jaynes-Cummings physics is not restricted to light-matter coupling below the ultrastrong limit. Among the many implications of this finding is that the system's ground state is not necessarily highly entangled, which is usually considered a hallmark of the ultrastrong-coupling regime.