  The purpose of this paper is to provide a brief review of some recent
developments in quantum feedback networks and control. A quantum feedback
network (QFN) is an interconnected system consisting of open quantum systems
linked by free fields and/or direct physical couplings. Basic network
constructs, including series connections as well as feedback loops, are
discussed. The quantum feedback network theory provides a natural framework for
analysis and design. Basic properties such as dissipation, stability, passivity
and gain of open quantum systems are discussed. Control system design is also
discussed, primarily in the context of open linear quantum stochastic systems.
The issue of physical realizability is discussed, and explicit criteria for
stability, positive real lemma, and bounded real lemma are presented. Finally
for linear quantum systems, coherent $H^\infty$ and LQG control are described.
