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Estimating the expectation values of spin-1/2 observables with finite resources

Brougham, Thomas and Andersson, Erika (2007) Estimating the expectation values of spin-1/2 observables with finite resources. Physical Review A, 76 (5). ISSN 1094-1622

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We examine the problem of estimating the expectation values of two observables when we have a finite number of copies of an unknown qubit state. Specifically we examine whether it is better to measure each of the observables separately on different copies or to perform a separable nonadaptive joint measurement of the observables on each copy. We find that joint measurements can sometimes provide an advantage over separate measurements, but only if we make estimates of an observable based solely on the results of measurements of that observable. If we instead use both sets of results to estimate each observable then we find that individual measurements will be better. Finally we consider estimating the expectation values of three complementary observables for an unknown qubit.