  We consider the optimal discrimination of nonorthogonal qubit states with
post-measurement information and provide an analytic structure of the optimal
measurements. We also show that there is always a null optimal measurement when
post-measurement information is given. Further, in discriminating four states
using post-measurement information, we analytically provide the optimal
probability of correct guessing and show that the uniqueness of optimal
measurement is equivalent to the non-existence of non-null optimal measurement
with post-measurement information.
