Phys. Rev. A 69, 010304(R) (2004) [4 pages]Collective versus local measurements in a qubit mixed-state estimation
We discuss the problem of estimating a qubit mixed state. We give the optimal estimation that can be inferred from any given set of measurements. For collective measurements and for a large number N of copies, we show that the error in the estimation varies as 1/N. For local measurements, we focus on the simpler case of states lying on the equatorial plane of the Bloch sphere. We show that the error using plain tomography varies as 1/N1/4, while our approach leads to an error proportional to 1/N3/4. © 2004 The American Physical Society URL:
http://link.aps.org/doi/10.1103/PhysRevA.69.010304
DOI:
10.1103/PhysRevA.69.010304
PACS:
03.67.-a, 03.65.Wj
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