{
  "annotation_id": "277e7d97-df0e-4423-bc09-34253ca04ec0",
  "date_created": "2026-03-02T18:01:42.142000Z",
  "date_modified": "2026-03-02T18:01:42.142000Z",
  "file_hash": "19ede34f7662562c56616e1fa9cadbcd1a807bc301cd7468ae198e0372e9de0c",
  "private": false,
  "record": {
    "abstract": "We describe in detail the theory underpinning the measurement of density\nmatrices of a pair of quantum two-level systems (``qubits\u0027\u0027). Our particular\nemphasis is on qubits realized by the two polarization degrees of freedom of a\npair of entangled photons generated in a down-conversion experiment; however\nthe discussion applies in general, regardless of the actual physical\nrealization. Two techniques are discussed, namely a tomographic reconstruction\n(in which the density matrix is linearly related to a set of measured\nquantities) and a maximum likelihood technique which requires numerical\noptimization (but has the advantage of producing density matrices which are\nalways non-negative definite). In addition a detailed error analysis is\npresented, allowing errors in quantities derived from the density matrix, such\nas the entropy or entanglement of formation, to be estimated. Examples based on\ndown-conversion experiments are used to illustrate our results.",
    "arxiv_id": "quant-ph/0103121",
    "authors": [
      "Daniel F. V. James",
      "Paul G. Kwiat",
      "William J. Munro",
      "Andrew G. White"
    ],
    "categories": [
      "quant-ph"
    ],
    "doi": "10.1103/PhysRevA.64.052312",
    "journal_ref": "Physical Review A 64, 052312 (2001)",
    "title": "On the Measurement of Qubits",
    "url": "https://arxiv.org/abs/quant-ph/0103121"
  },
  "schema_id": "dorsal/arxiv",
  "source": {
    "execution_id": "a9a3fb4c-82b1-4b88-b5d3-7846c616fe92",
    "id": "arXiv Dataset IDs",
    "type": "Model",
    "variant": "snapshot-2026-03-01",
    "version": "0.1.0"
  },
  "user_id": 1000002
}