  In conventional quantum nondemolition measurements, the interaction between
signal and probe preserves the measured variable. Alternatively, it is possible
to restore the original value of the variable by feedback. In this paper, we
describe a quantum nondemolition measurement of a quadrature component of the
light field using a feedback compensated beam splitter. The noise induced by
the vacuum port of the beam splitter is compensated by a linear feedback
resulting in an effective amplification of the observed variable. This
amplification is then be reversed by optical parametric amplification to
restore the original value of the field component.
