Invertible condition of quantum Fisher information matrix for a mixed   qubit
  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. 
Ping Yue and Li Ge and Qiang Zheng
quant-ph