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Optimal estimation of quantum dynamics

A. Acin, E. Jane, G. Vidal

quant-pharXiv:quant-ph/0012015

Abstract

We construct the optimal strategy for the estimation of an unknown unitary transformation U∈ SU(d). This includes, in addition to a convenient measurement on a probe system, finding which is the best initial state on which U is to act. When U∈ SU(2), such an optimal strategy can be applied to estimate simultaneously both the direction and the strength of a magnetic field, and shows how to use a spin 1/2 particle to transmit information about a whole coordinate system instead of only a direction in space.

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