  The principles are elaborated which underlie the applications of general
nonclassical states to communication and measurement systems. Relevant
classical communication concepts are reviewed. Communication and measurement
processes are compared. The possible advantages of nonclassical states in
classical information transfer are assessed. The significance of novel quantum
amplifiers and duplicators in communication is emphasized. A general approach
is developed for determining the ultimate accuracy limit in quantum measurement
systems. It is found that bandwidth or mode number is a most important
parameter and ultrahigh precision measurement is possible in systems with a
fixed energy but many modes. The problem of the standard quantum limit in
monitoring the position of a free mass is also addressed.
