Classification and Construction of Quantum Communication Systems

We consider Quantum Communication Systems (QCS's), introduced by Davies [10] on trace class operators, on general state spaces. These are different from the usual quantum information theory as they deal with input and output being continuous in time. We introduce the concept of a refinement of a QCS, offering the possibility of distinguishing particles, for instance according to their phase or location, and classify refinements of general QCS. We then introduce the notions of a bounded interaction rate and a bounded modulation and give, in special cases, a classification of QCS's satisfying both conditions. As this classification shows the existence of a class of QCS's being completely different from the examples studied by Davies, we are able to discuss the cases of an independent detector and a linear modulation. Received: 5 September 1995 / Accepted: 5 November 1996