  The purpose of this paper is to extend J.C. Willems' theory of dissipative
systems to the quantum domain. This general theory, which combines perspectives
from the quantum physics and control engineering communities, provides useful
methods for analysis and design of dissipative quantum systems. We describe the
interaction of the plant and a class of exosystems in general quantum feedback
network terms. Our results include an infinitesimal characterization of the
dissipation property, which generalizes the well-known Positive Real and
Bounded Real Lemmas, and is used to study some properties of quantum
dissipative systems. We also show how to formulate control design problems
using quantum network models, which implements Willems' `control by
interconnection' for open quantum systems. This control design formulation
includes, for example, standard problems of stabilization, regulation, and
robust control.

Quantum Physics