  We address the class of positive operator-valued measures (POVMs) for qubit
systems that are obtained by coupling the signal qubit with a probe qubit and
then performing a projective measurement on the sole probe system. These POVMs,
which represent the simplest class of qubit POVMs, depends on 3+3+2=8 free
parameters describing the initial preparation of the probe qubit, the Cartan
representative of the unitary coupling, and the projective measurement at the
output, respectively. We analyze in some details the properties of the POVM
matrix elements, and investigate their values for given ranges of the free
parameters. We also analyze in details the tradeoff between information and
disturbance for different ranges of the free parameters, showing, among other
things, that i) typical values of the tradeoff are close to optimality and ii)
even using a maximally mixed probe one may achieve optimal tradeoff.
