Downing C A, Toghill A J
Department of Physics and Astronomy, University of Exeter, Exeter, EX4 4QL, UK.
Sci Rep. 2022 Jul 8;12(1):11630. doi: 10.1038/s41598-022-15735-0.
The coupling between two or more objects can generally be categorized as strong or weak. In cavity quantum electrodynamics for example, when the coupling strength is larger than the loss rate the coupling is termed strong, and otherwise it is dubbed weak. Ultrastrong coupling, where the interaction energy is of the same order of magnitude as the bare energies of the uncoupled objects, presents a new paradigm for quantum physics and beyond. As a consequence profound changes to well established phenomena occur, for instance the ground state in an ultrastrongly coupled system is not empty but hosts virtual excitations due to the existence of processes which do not conserve the total number of excitations. The implications of ultrastrong coupling for quantum topological systems, where the number of excitations are typically conserved, remain largely unknown despite the great utility of topological matter. Here we reveal how the delicate interplay between ultrastrong coupling and topological states manifests in a one-dimensional array. We study theoretically a dimerized chain of two-level systems within the ultrastrong coupling regime, where the combined saturation and counter-rotating terms in the Hamiltonian are shown to play pivotal roles in the rich, multi-excitation effective bandstructure. In particular, we uncover unusual topological edge states, we introduce a flavour of topological state which we call an anti-edge state, and we reveal the remarkable geometric-dependent renormalizations of the quantum vaccum. Taken together, our results provide a route map for experimentalists to characterize and explore a prototypical system in the emerging field of ultrastrong quantum topology.
两个或多个物体之间的耦合通常可分为强耦合或弱耦合。例如，在腔量子电动力学中，当耦合强度大于损耗率时，这种耦合被称为强耦合，否则则被称为弱耦合。超强耦合是指相互作用能量与未耦合物体的裸能量具有相同的数量级，它为量子物理学及其他领域呈现了一种新的范式。结果，一些已确立的现象发生了深刻变化，例如，在超强耦合系统中，基态并非空态，而是由于存在不守恒激发总数的过程而存在虚拟激发。尽管拓扑物质有很大用途，但超强耦合对量子拓扑系统（其中激发数通常是守恒的）的影响在很大程度上仍然未知。在这里，我们揭示了超强耦合与拓扑态之间微妙的相互作用如何在一维阵列中体现。我们从理论上研究了超强耦合 regime 内二能级系统的二聚化链，其中哈密顿量中的组合饱和项和反向旋转项在丰富的多激发有效能带结构中起着关键作用。特别是，我们发现了不寻常的拓扑边缘态，引入了一种我们称之为反边缘态的拓扑态，并揭示了量子真空显著的几何相关重整化。综上所述，我们的结果为实验人员提供了一个路线图，以表征和探索超强量子拓扑新兴领域中的一个原型系统。
