  We previously extended Luenberger's approach for observer design to the
quantum case, and developed a class of coherent observers which tracks linear
quantum stochastic systems in the sense of mean values. In light of the fact
that the Luenberger observer is commonly and successfully applied in classical
control, it is interesting to investigate the role of coherent observers in
quantum feedback. As the first step in exploring observer-based coherent
control, in this paper we study pole-placement techniques for quantum systems
using coherent observers, and in such a fashion, poles of a closed-loop quantum
system can be relocated at any desired locations. In comparison to classical
feedback control design incorporating the Luenberger observer, here direct
coupling between a quantum plant and the observer-based controller are allowed
to enable a greater degree of freedom for the design of controller parameters.
A separation principle is presented, and we show how to design the observer and
feedback independently to be consistent with the laws of quantum mechanics. The
proposed scheme is applicable to coherent feedback control of quantum systems,
especially when the transient dynamic response is of interest, and this issue
is illustrated in an example.
