  Using Gardiner and Collet's input-output model and the concept of cascade
system, we determine the filtering equation for a quantum system driven by
chosen non-classical states of light. The quantum system and electromagnetic
field are described by making use of quantum stochastic unitary evolution. We
consider two examples of the non-classical states of the field: a combination
of vacuum and single photon states and a mixture of two coherent states. We
describe the stochastic evolution conditioned on the results of the photon
counting and quadrature measurements.
