  We explore a field theoretical approach to quantum computing and control.
This book consists of three parts. The basics of systems theory and field
theory are reviewed in Part I. In Part II, a gauge theory is reinterpreted from
a systems theoretical perspective and applied to the formulation of quantum
gates. Then quantum systems are defined by introducing feedback to the gates.
In Part III, quantum gates and systems are reformulated from a quantum field
theoretical perspective using S-matrices. We also discuss how gauge fields are
related to feedback.
