ABSTRACT
In this appendix we state the axioms upon which quantum mechanics is founded. It will be sufficient for purposes of this book to restrict our dis cussion to the non-relativistic theory. In quantum mechanics, the primitive undefined concepts are physical system, observable, and state. A physical sys tem will be considered to be any sufficiently isolated thing, say an electron, a molecule, or a photon. An observable will be identified with a measurable property of a physical system, say energy or z-component of spin. The state of a physical system proves to be a trickier concept in quantum mechanics than in classical mechanics. Subtleties arise when considering the state of a composite physical system. In particular, states exist for a bipartite physical system in which neither of the subsystems is in a definite state. Such states are known as entangled states (Section 1.3.5) and are inherently quantum mechanical. Even in cases where a physical system can be described as being in a state, two classes of state are possible: pure and mixed. These two types of states will defined later in this appendix.
