Estimating multiparamter simultaneously as precise as possible is an important goal of quantum metrolo- gy. As a first step to this end, here we give a condition determining whether two arbitrary parameters can be estimated simultaneously for a qubit in the mixed state. An application of this condition is shown.
@article{arxiv.1601.05866,
title = {Invertible condition of quantum Fisher information matrix for a mixed qubit},
author = {Ping Yue and Li Ge and Qiang Zheng},
journal= {arXiv preprint arXiv:1601.05866},
year = {2016}
}