  The realization of transfer functions of Linear Quantum Stochastic Systems
(LQSSs) is an issue of fundamental importance for the practical applications of
such systems, especially as coherent controllers for other quantum systems. In
this paper, we review two realization methods proposed by the authors in [1],
[2], [3], [4]. The first one uses a cascade of a static linear quantum-optical
network and single-mode optical cavities, while the second uses a feedback
network of such cavities, along with static linear quantum-optical networks
that pre- and post-process the cavity network inputs and outputs.
