Coherent observer engineering for protecting quantum information

In this paper, we extend the applicability of coherent quantum observers to enable the generation of decoherence free subsystem which can be used for quantum information storage. In particular, a general quantum linear passive system can be equipped with a decoherence free subsystem by placing some of its poles on the imaginary axis via engineered observer-based controller. The controller design for the case of optical cavity systems is illustrated in the context of examples.