School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
Nat Commun. 2019 Jan 30;10(1):499. doi: 10.1038/s41467-018-08101-0.
Ultrastrong-coupling between two-level systems and radiation is important for both fundamental and applied quantum electrodynamics (QED). Such regimes are identified by the breakdown of the rotating-wave approximation, which applied to the quantum Rabi model (QRM) yields the apparently less fundamental Jaynes-Cummings model (JCM). We show that when truncating the material system to two levels, each gauge gives a different description whose predictions vary significantly for ultrastrong-coupling. QRMs are obtained through specific gauge choices, but so too is a JCM without needing the rotating-wave approximation. Analysing a circuit QED setup, we find that this JCM provides more accurate predictions than the QRM for the ground state, and often for the first excited state as well. Thus, Jaynes-Cummings physics is not restricted to light-matter coupling below the ultrastrong limit. Among the many implications is that the system's ground state is not necessarily highly entangled, which is usually considered a hallmark of ultrastrong-coupling.
两能级系统与光的强耦合在基础和应用量子电动力学（QED）中都很重要。这种模式是通过旋转波近似的失效来确定的，应用于量子拉比模型（QRM）会得到明显不那么基础的 Jaynes-Cummings 模型（JCM）。我们表明，当将物质系统截断到两个能级时，每个规范给出了不同的描述，对于强耦合，这些描述的预测差异很大。QRMs 通过特定的规范选择得到，但不需要旋转波近似也可以得到 JCM。分析一个电路 QED 装置，我们发现这个 JCM 比 QRM 更准确地预测基态，而且通常对第一激发态也是如此。因此，Jaynes-Cummings 物理并不局限于低于强耦合极限的光物质耦合。其中一个重要的推论是，系统的基态不一定是高度纠缠的，这通常被认为是强耦合的一个标志。
