On the Measurement of Qubits

Details On the Measurement of Qubits On the Measurement of Qubits

2001-03-21

arxiv.org/abs/quant-ph/0103121v1

Physical Review A 64, 052312 (2001)

Physics

Quantum Physics

23 pages, 3 figures, submitted to Phys Rev A

Scientific paper

10.1103/PhysRevA.64.052312

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.

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