Embracing gauge-freedom in ultrastrong-coupling quantum electrodynamics

从原子尺度以上，几乎所有量子系统的基本描述都是由量子电动力学（QED）提供的。在分子和固态腔和电路QED系统中遇到的越来越强的光-物质相互作用为探索基本量子现象提供了一个重要的平台，从而导致新的发现和对物理学基础的新见解。例如，尽管术语“光子”（意思是光粒子）被普遍使用，但光子的真实性质仍然难以捉摸，以至于整个会议都致力于试图建立“什么是光子”？强的光-物质相互作用为光和物质的基本性质提出了许多新的问题，因为在这种情况下，它们之间的边界变得模糊。我们对当代物理学的理解是基于这样一个假设，即较大的（复合）系统可以分为称为子系统的组成部分。例如，一个原子和它的环境都是原子-环境整体的子系统。QED在量子尺度上描述了这样的系统，但它可以用许多不同的形式来表达（称为规范自由）。每一种形式（即规范选择）都提供了一个不同的理论划分，将整个系统划分为组成光和物质的子系统。此外，该理论本身并不赞成使用任何一个子系统划分。虽然QED是一个非常完善的理论，在传统的弱耦合制度的原子光物质物理学，不同的子系统划分对应于不同形式的QED变得越来越不同的系统内的相互作用的强度增加。例如，“光子”的可用定义在强耦合机制中可能会有很大变化。因此，为了准确地预测在涉及强光-物质相互作用的实验中由光探测器记录的点击次数，我们必须确定光子的无限可能定义中的哪一个实际上对应于探测器记录的内容。该方案的目的是提供对光、物质及其在强耦合体系中相互作用的新的基本认识。为此，一个原始的和更强大的QED公式将被开发出来，它通过允许子系统之间的边界变化来释放它的全部通用性。这将通过开放量规的选择来实现，而不是从一开始就固定它。该提案具有产生对基本物理理解具有重要意义的变革性结果的巨大潜力。

The underlying descriptions of nearly all quantum systems, from the atomic-scale upwards, are provided by quantum electrodynamics (QED). The increasingly strong light-matter interactions encountered in molecular and solid-state cavity and circuit QED systems offer an important platform for probing fundamental quantum phenomena resulting in new discoveries and new insight into the foundations of physics. For example, although the term "photon" (meaning a light particle) is commonly used, the true nature of photons remains elusive to the extent that entire conferences are dedicated to trying to establish "what is a photon"? Strong light-matter interactions open up a wealth of new questions concerning the fundamental nature of light and matter, because in this regime the boundary between them becomes blurred. Our understanding of contemporary physics is predicated on the assumption that larger (composite) systems can be divided into constituent parts known as subsystems. For example, an atom and its environment are both subsystems of the atom-environment whole. QED describes such systems at the quantum scale, but it can be expressed in many different forms (known as gauge freedom). Each form (i.e. gauge choice) provides a different theoretical division of the overall system into constituent light and matter subsystems. Moreover, the theory does not itself favour the use of any one subsystem division over any other. While QED is an exceptionally well-established theory within the conventional weak-coupling regime of atomic light-matter physics, distinct subsystem divisions corresponding to distinct forms of QED become increasingly different as the interactions within the system increase in strength. For example, the available definitions of a "photon" can vary significantly in the strong-coupling regime. Therefore, to accurately predict the number of clicks registered by a photo-detector in an experiment involving strong light-matter interactions, we must determine which of the infinite possible definitions of photon actually corresponds to what is registered by the detector. The objective of this proposal is to provide new fundamental understanding of light, matter and their interaction in the strong-coupling regime. To this end an original and more powerful formulation of QED will be developed, which unlocks its full generality by allowing the boundaries drawn between subsystems to vary. This will be achieved by leaving the choice of gauge open, rather than fixing it from the outset. The proposal has significant potential to produce transformative results of importance for fundamental physical understanding.