  We show that it is possible to construct closed quantum systems governed by a
bilinear Hamiltonian depending on an arbitrary input signal. This is achieved
by coupling the system to a quantum input field and performing a feedback of
the output field back into the system to cancel out the stochastic effects,
with the signal being added to the field between these events and later
subtracted. Here we assume the zero time delay limit between the various
connections and operations.
