{
  "id": "quant-ph/0103121",
  "version": "v1",
  "published": "2001-03-21T00:47:10.000Z",
  "updated": "2001-03-21T00:47:10.000Z",
  "title": "On the Measurement of Qubits",
  "authors": [
    "Daniel F. V. James",
    "Paul G. Kwiat",
    "William J. Munro",
    "Andrew G. White"
  ],
  "comment": "23 pages, 3 figures, submitted to Phys Rev A",
  "journal": "Physical Review A 64, 052312 (2001)",
  "doi": "10.1103/PhysRevA.64.052312",
  "categories": [
    "quant-ph"
  ],
  "abstract": "We describe in detail the theory underpinning the measurement of density matrices of a pair of quantum two-level systems (``qubits''). Our particular emphasis is on qubits realized by the two polarization degrees of freedom of a pair of entangled photons generated in a down-conversion experiment; however the discussion applies in general, regardless of the actual physical realization. Two techniques are discussed, namely a tomographic reconstruction (in which the density matrix is linearly related to a set of measured quantities) and a maximum likelihood technique which requires numerical optimization (but has the advantage of producing density matrices which are always non-negative definite). In addition a detailed error analysis is presented, allowing errors in quantities derived from the density matrix, such as the entropy or entanglement of formation, to be estimated. Examples based on down-conversion experiments are used to illustrate our results.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2001-03-21T00:47:10.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "measurement",
      "down-conversion experiment",
      "density matrix",
      "quantum two-level systems",
      "maximum likelihood technique"
    ],
    "tags": [
      "journal article"
    ],
    "publication": {
      "publisher": "APS",
      "journal": "Phys. Rev. A"
    },
    "note": {
      "typesetting": "TeX",
      "pages": 23,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}