Gauge freedom in quantum electrodynamics (QED) outside of textbook regimes is reviewed. It is emphasized that QED subsystems are defined relative to a choice of gauge. Each definition uses different gauge-invariant observables. This relativity is eliminated only if a sufficient number of Markovian and weak-coupling approximations are employed. All physical predictions are gauge invariant, including subsystem properties such as photon number and entanglement. However, subsystem properties naturally differ for different physical subsystems. Gauge ambiguities arise not because it is unclear how to obtain gauge-invariant predictions, but because it is not always clear which physical observables are the most operationally relevant. The gauge invariance of a prediction is necessary but not sufficient to ensure its operational relevance. It is shown that, in controlling which gauge-invariant observables are used to define a material system, the choice of gauge affects the balance between the material system’s localization and its electromagnetic dressing. Various implications of subsystem gauge relativity for deriving effective models, for describing time-dependent interactions, for photodetection theory, and for describing matter within a cavity are reviewed.

中文翻译：

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规范自由度对非相对论量子电动力学的影响

回顾了教科书体系之外的量子电动力学（QED）中的规范自由度。需要强调的是，QED 子系统的定义与规范的选择相关。每个定义都使用不同的规范不变可观测量。仅当采用足够数量的马尔可夫和弱耦合近似时，这种相对性才会被消除。所有物理预测都是规范不变的，包括子系统属性，例如光子数和纠缠。然而，不同物理子系统的子系统属性自然不同。规范模糊性的出现并不是因为不清楚如何获得规范不变的预测，而是因为并不总是清楚哪些物理可观测量与操作最相关。预测的规范不变性是必要的，但不足以确保其操作相关性。结果表明，在控制使用哪些规范不变的可观测量来定义材料系统时，规范的选择会影响材料系统的定位与其电磁修整之间的平衡。回顾了子系统规范相对论对于推导有效模型、描述瞬态相互作用、光电探测理论以及描述腔内物质的各种影响。