Estimation of unitary quantum operations

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Abstract
In this paper the problem of optimally estimating an unknown unitary quantum operation is treated. A comparison is made between separable and non-separable measurements and it is shown that non-separable measurements are always better at least by a factor of two. It is also shown that if separable measurements are used, classical communication does not improve the estimation. In two dimensions it is also shown that measuring one of the bell states against the other three is as good as any separable measurement and has the advantage of having only two outcomes.

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