MPS-RR 1998-13
August 1998

An Example of Non-Attainability of Expected Quantum Information

Ole E. Barndorff-Nielsen

R. D. Gill

Abstract

Braunstein and Caves showed how quantum information can be used to define a metric for quantum states, relying on attainability of a quantum information bound. We show that the bound is not generally attainable. But a two-stage procedure of repeated measurements achieves the bound in the limit. We connect to the question of whether a generalised measurement on n independent copies of a quantum system can yield more information than n separate measurements. Separate measurements are in the limit as informative as joint, and measurement of classical observables suffices.

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