Measurement of qubits

Daniel F.V. James, Paul G. Kwiat, William J. Munro, Andrew G. White

Research output: Contribution to journalArticlepeer-review


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 that 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.

Original languageEnglish (US)
Pages (from-to)15
Number of pages1
JournalPhysical Review A - Atomic, Molecular, and Optical Physics
Issue number5
StatePublished - 2001

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics


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