{
  "id": "1803.04302",
  "version": "v1",
  "published": "2018-03-12T15:12:59.000Z",
  "updated": "2018-03-12T15:12:59.000Z",
  "title": "Indefinite Causal Order in a Quantum Switch",
  "authors": [
    "K. Goswami",
    "C. Giarmatzi",
    "M. Kewming",
    "F. Costa",
    "C. Branciard",
    "J. Romero",
    "A. G. White"
  ],
  "categories": [
    "quant-ph"
  ],
  "abstract": "In quantum mechanics events can happen in no definite causal order: in practice this can be verified by measuring a causal witness, in the same way that an entanglement witness verifies entanglement. Indefinite causal order can be observed in a quantum switch, where two operations act in a quantum superposition of the two possible orders. Here we realise a photonic quantum switch, where polarisation coherently controls the order of two operations, $\\hat{A}$ and $\\hat{B}$, on the transverse spatial mode of the photons. Our setup avoids the limitations of earlier implementations: the operations cannot be distinguished by spatial or temporal position. We show that our quantum switch has no definite causal order, by constructing a causal witness and measuring its value to be 18 standard deviations beyond the definite-order bound.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2018-03-12T15:12:59.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "indefinite causal order",
      "causal witness",
      "entanglement witness verifies entanglement",
      "operations",
      "quantum mechanics events"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}