## Article

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

##### Issue: Volume 70 Series A Part 1 Year 2008
###### Abstract
This paper concerns the design problem of choosing the measurement that provides the maximum Fisher information for the unknown parameter of a quantum system. We 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 Helstrom 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).