Περίληψη 
This thesis reviews the fundamental principles of quantum optics, focusing on lightmatter interactions. The dynamics of twolevel systems under the influence of classical continuous and pulsed fields, as well as quantized fields are examined. Additionally, the phenomenon of reversible spontaneous emission in a cavity is studied, shedding light on the emission characteristics of atoms interacting with the quantized field. Furthermore, the coupling of the atomfield system with a reservoir is explored, analyzing the effects of the reservoir on the system's dynamics which results in irreversible spontaneous emission. Through theoretical analysis and numerical simulations, this research deepens our understanding of the intricate interplay between light and matter at both the classical and the quantum level, one of the most essential concepts in quantum optics.
Chapter 1 serves as a brief review of quantum mechanics, setting the tools for the subsequent chapters. It begins with a detailed overview of quantum mechanics, including its postulates and mathematical formalism. The coupling of radiation fields with atomic electrons and the concept of electric dipole interactions are then discussed, highlighting their importance in understanding the dynamics of atomfield interactions. The chapter further explores the Schrödinger, Heisenberg, and interaction pictures, which provide alternative perspectives for analyzing quantum systems. Timedependent perturbation theory is introduced as a useful tool for studying the behavior of quantum systems under external influences. Finally, this chapter also delves into the specific case of atomfield interactions involving twolevel atoms. The dynamics of such interactions are investigated, considering the atomfield coupling and the resulting effects on the atomic states. The concept of the density operator is introduced, providing a mathematical framework for describing the behavior of quantum systems.
Building upon the foundational knowledge established in Chapter 1, Chapter 2 focuses on field quantization. The singlemode field quantization is discussed in detail, highlighting the quantized nature of field excitations and the associated creation and annihilation operators. Quantum fluctuations of a singlemode field are examined, explaining the inherent uncertainty and fluctuations present in quantum systems. The study extends to multimode fields, coherent states, and the behavior of singlemode fields in thermal equilibrium.
Chapter 3 investigates the interaction of twolevel atoms with monochromatic classical fields. The dynamics of this interaction are analyzed, considering both continuous and pulsed classical fields. The effects of the classical field on the atomic states and the resulting phenomena are explored, providing an understanding about the dynamics of lightmatter interactions in classical regimes.
Chapter 4 delves into the interaction of twolevel atoms with quantized fields. The JaynesCummings model is introduced as a fundamental theoretical framework for describing the interaction between a twolevel atom and a quantized field. The concept of dressed states is explored, revealing the modification of atomic energy levels due to the coupling with the quantized field. The behavior of the twolevel atom interacting with a field in number (Fock) and coherent state is investigated, elucidating the connection between the field's quantum state and its interaction with the system. In Chapter 5, the focus shifts to the coupling of a twolevel system with a reservoir. The interaction of a twolevel atom with a continuum of modes gives rise to irreversible spontaneous emission, a fundamental process in which an excited atom emits a photon and returns to its ground state. The chapter explores the theory of spontaneous emission in free space, considering the density of states and its impact on the emission spectrum. The WeisskopfWigner theory provides a theoretical framework for analyzing spontaneous emission dynamics in detail.
Chapter 6 delves into the broader context of systemreservoir interactions. It begins by investigating the harmonic oscillator coupled to a reservoir of harmonic oscillators, which serves as a model for describing the dynamics of atomfield interactions in the presence of a surrounding environment. The spontaneous decay of a twolevel atom is then examined, revealing the intricate interplay between the atom and its environment which results in the decay dynamics.
By exploring the contents outlined in this thesis, we aim to deepen our understanding of atomfield interactions from both classical and quantum perspectives. The theoretical frameworks and mathematical tools presented provide a solid foundation for analyzing and explaining the behavior of twolevel systems in the presence of external fields and reservoirs.
