Quantum Coherence and Measurement Theory

abstract

The feature of quantum mechanics which most distinguishes it from classical mechanics is the coherent superposition of distinct physical states.Many of the less intuitive aspects of the quantum theory can be traced to this feature. Does the superposition principle operate on macroscopic scales? The famous Schroedinger cat argument highlights problems of interpretation were macroscopic superposition states allowed. In this chapter we discuss schemes to produce and detect superposition states in an optical context. We shall show that such states are very fragile in the presence of dissipation and rapidly collapse to a classical mixture exhibiting no unusual interference features.

The superposition principle is also the source of the “problem of measurement” in quantum mechanics.We do not attempt to present a solution to this problem here. Rather we show how the effect of the environment on superposition states enables a consistent description of the measurement process to be given and which avoids some of the problems inherent in previous approaches.