Coherent Light

In this chapter, we explore the concept of light coherence. We examine its experimental and theoretical aspects in classical physics and address the challenges of applying these principles to quantum systems. In the quantum domain, we delve into the quantum nature of light through the Fock states, which are characterized by fixed photon numbers. These states provide insights into light’s quantum states but become complex with multiple photons. To streamline the study of light coherence, we discuss a powerful alternative, the eigenstates of the photon annihilation operator, known as coherent states. These states present a more convenient and tractable approach for investigating light coherence. Notably, when the photon count is large, the fluctuation characteristics of these coherent states closely resemble those of classical electromagnetic waves, simplifying the analysis and bridging the quantum-classical divide.