{
  "id": "2009.10662",
  "version": "v1",
  "published": "2020-09-22T16:32:47.000Z",
  "updated": "2020-09-22T16:32:47.000Z",
  "title": "Implications of gauge-freedom for non-relativistic quantum electrodynamics",
  "authors": [
    "Adam Stokes",
    "Ahsan Nazir"
  ],
  "comment": "Review article. 57 pages including appendix",
  "categories": [
    "quant-ph"
  ],
  "abstract": "We review gauge-freedom in quantum electrodynamics (QED) outside of textbook regimes. We emphasise that QED subsystems are defined {\\em relative} to a choice of gauge. Each definition uses different gauge-invariant observables. We show that this relativity is only eliminated 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. We show 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 localisation and its electromagnetic dressing. We review 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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2020-09-22T16:32:47.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "non-relativistic quantum electrodynamics",
      "implications",
      "gauge-invariant observables",
      "subsystem properties",
      "material systems localisation"
    ],
    "tags": [
      "review article"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 57,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}