{
  "id": "1907.03464",
  "version": "v1",
  "published": "2019-07-08T09:07:21.000Z",
  "updated": "2019-07-08T09:07:21.000Z",
  "title": "Quantum states are equivalence classes of stable information, not density operators",
  "authors": [
    "Bradley A. Foreman"
  ],
  "comment": "6 pages",
  "categories": [
    "quant-ph",
    "physics.hist-ph"
  ],
  "abstract": "The Copenhagen interpretation of quantum mechanics, which first took shape in Bohr's landmark 1928 paper on complementarity, remains an enigma. Although many physicists are skeptical about the necessity of Bohr's philosophical conclusions, his pragmatic message about the importance of the whole experimental arrangement is widely accepted. It is, however, generally also agreed that the Copenhagen interpretation has no direct consequences for the mathematical structure of quantum mechanics, aside from the exclusion of such things as hidden variables. Here I show that the application of Bohr's main concepts of complementarity to the subsystems of a closed system requires a change in the definition of the quantum state. The appropriate definition is not as a density operator but as an equivalence class of density operators. The specific equivalence class is determined by a stability criterion taken from decoherence theory. The central issue is the extraction of information from experiments, as stressed by Bohr. The theory developed here can be regarded as an inevitable consequence of merging Bohr's concepts of complementarity with the Everett formulation of quantum mechanics. The resulting definition of the quantum state is, however, relevant for all interpretations of quantum mechanics, because the extraction of information from experiments is an operational question that must be considered in any interpretation.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-07-08T09:07:21.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "quantum state",
      "density operator",
      "quantum mechanics",
      "stable information",
      "copenhagen interpretation"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 6,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}