ABSTRACT
Quantum systems range from the traditional microscopic ones like electron and atoms to macroscopic ones like superconductors. There are also quantum fields. This chapter sets out one general properties of these systems. For a classical particle, such a sequence does converge to give a value for the instantaneous velocity with arbitrary accuracy. For a quantum particle, the situation is entirely different. It is accepted that a quantum system, say an electron, has an objective existence. At the outset we assume that a measurement operation exists to produce a value of any given observable of the system. This assumption is built into the concept of observables which are physical quantities which can be measured to produce a value. Since a quantum particle in a general state may not have an objective value of a given observable independent of measurement it is useful to distinguish two kinds of values: possessed values and measured values.
