  We suggest an interferometric scheme assisted by squeezing and linear
feedback to realize the whole class of field-quadrature quantum nondemolition
measurements, from Von Neumann projective measurement to fully non-destructive
non-informative one. In our setup, the signal under investigation is mixed with
a squeezed probe in an interferometer and, at the output, one of the two modes
is revealed through homodyne detection. The second beam is then
amplitude-modulated according to the outcome of the measurement, and finally
squeezed according to the transmittivity of the interferometer. Using strongly
squeezed or anti-squeezed probes respectively, one achieves either a projective
measurement, i.e. homodyne statistics arbitrarily close to the intrinsic
quadrature distribution of the signal, and conditional outputs approaching the
corresponding eigenstates, or fully non-destructive one, characterized by an
almost uniform homodyne statistics, and by an output state arbitrarily close to
the input signal. By varying the squeezing between these two extremes, or
simply by tuning the internal phase-shift of the interferometer, the whole set
of intermediate cases can also be obtained. In particular, an optimal quantum
nondemolition measurement of quadrature can be achieved, which minimizes the
information gain versus state disturbance trade-off.
