|Title||Collective versus local measurements in a qubit mixed-state estimation|
|Publication Type||Journal Article|
|Year of Publication||2004|
|Authors||Bagan, E, Baig, M, Muñoz-Tapia, R, Rodriguez, A|
|Journal||Physical Review A (Atomic, Molecular, and Optical Physics)|
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.