Embracing gauge-freedom in ultrastrong-coupling quantum electrodynamics

几乎所有量子系统的基本描述，从原子尺度以上，都由量子电动力学(QED)提供。在分子和固态腔和电路QED系统中遇到的日益强烈的光-物质相互作用为探索基本的量子现象提供了一个重要的平台，导致了新的发现和对物理学基础的新认识。例如，尽管术语“光子”(指轻粒子)被普遍使用，但光子的真正本质仍然难以捉摸，以至于整个会议都致力于试图建立“什么是光子”？强烈的光-物质相互作用打开了大量关于光和物质的基本性质的新问题，因为在这种体制下，它们之间的边界变得模糊。我们对当代物理学的理解是建立在这样一种假设之上的，即较大的(复合)系统可以被分成称为子系统的组成部分。例如，原子及其环境都是原子-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示(称为规范自由)。每种形式(即标准选择)提供了一个不同的理论划分，将整个系统分为组成光和物质的子系统。此外，该理论本身并不支持使用任何一个子系统部门而不是其他任何部门。虽然量子电动力学在原子轻物质物理的常规弱耦合机制中是一个非常成熟的理论，但随着系统内相互作用强度的增加，与不同形式的量子电动力学相对应的不同的子系统划分变得越来越不同。例如，在强耦合状态下，现有的“光子”定义可能有很大的不同。因此，为了准确预测光探测器在涉及强光物质相互作用的实验中登记的点击次数，我们必须确定无限可能的光子定义中的哪一个实际上与探测器登记的内容相对应。这一提议的目的是提供对光、物质及其在强耦合系统中的相互作用的新的基本理解。为此，将开发一种原创的、更强大的QED公式，通过允许子系统之间的边界变化来解锁其全部共性。这将通过保留量规的选择余地来实现，而不是从一开始就固定它。这项提议有很大潜力产生对基本物理理解具有重要意义的变革性结果。

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.