  A natural formulation of the theory of quantum measurements in continuous
time is based on quantum stochastic differential equations
(Hudson-Parthasarathy equations). However, such a theory was developed only in
the case of Hudson-Parthasarathy equations with bounded coefficients. By using
some results on Hudson-Parthasarathy equations with unbounded coefficients, we
are able to extend the theory of quantum continuous measurements to cases in
which unbounded operators on the system space are involved. A significant
example of a quantum optical system (the degenerate parametric oscillator) is
shown to fulfill the hypotheses introduced in the general theory.
