  In this article we discuss a general information extraction scheme to gain
knowledge of the state and the amount of decoherence based on indirect
continuous measurement. The purpose of this information extraction is to
determine the feedback in order to control decoherence based on an ``output
equation", described by a bilinear form. An interacting bilinear form of
control system is used instead of master equation to analyze the dynamic
properties of the system. The analysis of continuous measurement on a
decohering quantum system has not been extensively studied before.
