Control of decoherence in open quantum systems using feedback

Quantum feedback is assuming increasingly im- portant role in quantum control and quantum information processing. In this work we analyze the application of such feedback techniques in eliminating decoherence in open quan- tum 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.