Wave packet spreading: Temperature and squeezing effects with applications to quantum measurement and decoherence

A localized free particle is represented by a wave packet and its motion is discussed in most quantum mechanics textbooks. Implicit in these discussions is the assumption of zero temperature. We discuss how the effects of finite temperature and squeezing can be incorporated in an elementary manner. The results show how the introduction of simple tools and ideas can bring the reader into contact with topics at the frontiers of research in quantum mechanics. We discuss the standard quantum limit, which is of interest in the measurement of small forces, and decoherence of a mixed ("Schrodinger cat") state, which has implications for current research in quantum computation, entanglement, and the quantum-classical interface. (C) 2002 American Association of Physics Teachers.