  We study the fidelity susceptibility of two SU(2)-invariant reduced density
matrices. Due to the commuting property of these matrices, analytical results
for reduced fidelity susceptibility are obtained and can be applied to study
quantum phase transitions in SU(2)-invariant systems. As an example, we analyze
the quantum criticality of the spin-one bilinear-biquadratic model via the
fidelity approach.
