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Quantum universal detectors

G. M. D'Ariano, P. Perinotti, M. F. Sacchi

quant-pharXiv:quant-ph/0306025

Abstract

We address the problem of estimating the expectation value <O> of an arbitrary operator O via a universal measuring apparatus that is independent of O, and for which the expectation values for different operators are obtained by changing only the data-processing. The ``universal detector'' performs a joint measurement on the system and on a suitably prepared ancilla. We characterize such universal detectors, and show how they can be obtained either via Bell measurements or via local measurements and classical communication between system and ancilla.

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