  Quantum feedback is assuming increasingly important role in quantum control
and quantum information processing. In this work we analyze the application of
such feedback techniques in eliminating decoherence in open quantum systems. In
order to apply such system theoretic methods we first analyze the invariance
properties of quadratic forms which corresponds to expected value of a
measurement and present conditions for decouplability of measurement outputs of
such time-varying open quantum systems from environmental effects.

Quantum Physics