Introduction

Abstract

The notion of coherent states (CS¹) is rooted in quantum physics and its relationship to classical physics. The term ‘coherent’ itself originates in the current language of quantum optics (for instance, coherent radiation, sources emitting coherently, etc.). It was introduced in the 1960s by Glauber [145], ² one of the founding fathers of the theory of CS, together with Klauder [189] and Sudarshan [Kl1, 272], in the context of a quantum optical description of coherent light beams emitted by lasers. Since then, coherent states have pervaded nearly all branches of quantum physics — quantum optics, of course, but also nuclear, atomic, and solid-state physics, quantum electrodynamics (the infrared problem), quantization and dequantization problems and path integrals, just to mention a few. It has been said, even convincingly [191], that “coherent states are the natural language of quantum theory!”