  We give a complete characterization for pure quantum measurements, i.e., for
POVMs which are extremals in the convex set of all POVMs. Such measurements are
free from classical noise. The characterization is valid both in discrete and
continuous cases, and also in the case of an infinite Hilbert space. We show
that sharp measurements are clean, i.e. they cannot be irreversibly connected
to another POVMs via quantum channels and thus they are free from any
additional quantum noise. We exhibit an example which demonstrates that this
result could also be approximately true for pure measurements.
