A Note on Fisher-Helstrom Information Inequality in Pure State Models

摘要

This paper concerns the design problem of choosing the measurement that provides the maximum Fisher information for the unknown parameter of a quantum system.rnWe show that when the system under investigation is described by a one-parameter n-dimensional pure state model an optimal measurement exists, such that Fisher information attains the upper bound constituted by Hel-strom information. A characterisation theorem and two strategies of implementations are derived and discussed. These results generalise to n-dimensional spaces those obtained for n = 2 by Barndorff-Nielsen and Gill (2000).