The Research Of Quantum Decoherence Of Atom-cavity Systems

Quantum mechanics is an important theory in the process of scientific development of the 20 th century. As the quantum-mechanics based new fields arisen, such as quantum computation and quantum information, quantum theory plays a more and more important role in the progress of modern society. Using the quantum theory to study and design quantum devices and then make quantum instruments and machines has become an important research interest of both fundamental theories and experimental applications. Atom-cavity system, due to its characteristics of high quality,easy preparation, become an important platform for realizing these researches. In recent years, high precision quantum measurements based on these properties of atom-cavity system has become a new research focus. One key technique question of precision quantum measurement or quantum metrology is quantum decoherence which correspondes to quantum coherence the essential difference between quantum mechanics and classical mechanics. However, the interaction between the quantum systems and the outside world will lead to the quantum decoherence of the quantum systems inevitably.Besides, the quantum decoherence has critical significance in the understanding of the transition from quantum world into classic one.This paper mainly studies the quantum decoherence of atom-field systems. Based on these research, we shed the light on the theoretical technics for making quantum devices and quantum instruments. Furthermore, we introduce the important applications of these theories in quantum metrology and correspond fields of precision measurement. This thesis consists of eight chapters and the main contents are given in Chapter 3-6.In Chapters 1 and 2, a briefly introduction of the background and progress of our researches is made, and the history of quantum decoherence, squeezed vacuum state, quasimode is reviewed. The concept and principle, such as density matrix, decoherence, quantization of radiation field, squeezed state and spontaneous emission are presented in detail.In Chapter 3, the decoherence dynamics of a two-level atom coupled to vacuum electromagnetic field is investigated. We calculate the decoherence factor, and the effect of spontaneous emission on the spatial quantum decoherence of the atom is also studied. The result shows that when there is a moving atoms in the cavity field, the coupling between the field and the internal state of atoms will induce the spontaneous emission of atom, and bring decoherence to the atomic mass center motion. Furthermore, the influences of the decoherence factor to some quantumphenomena are also revealed.In Chapter 4, the effect of the interaction between a single two-level atom and vacuum electromagnetic on the quantum decoherence of the motion of atomic mass center. And we analyze the influences of the atomic transition and atomic spontaneous emission on the motion coherence of atomic mass center. The result shows that the spontaneous emission of atom brings quantum entanglement between the atomic spatial degrees of freedom and the vacuum field, and therefore break the coherence of the motion of mass center. Therefore, we can establish some new theory to clarify how to ensure the reliability of the quantum device by modifying its parameters.In Chapter 5, we study the dynamic dissipation of an atom which is placed in a one-dimensional dissipative cavity. We calculate the dynamic decay rate of the atom and discuss the effect of detuning of the atom and the cavity mode as well as the size of the cavity on the atomic decay rate, find some new physics results based on the quasi-mode theory. The results show that the detuning between the atomic and field mode in not the same in different atomic decay rate, the greater the amount of detuning atomic decay rate is, the smaller the detuning is. And the size of the cavity affects the decay rate atoms, with the length of the cavity is increased, the atomic decay rate decreases.In Chapter 6, we study the influence of mechanical oscillator on the state of atom in coupled system. By consider the atom decoherence in coupled mechanical oscillator cavity system, we found that if the entire system evolves without any dissipations and external drives, there is no atom decoherence, the mechanical vibration of the mechanical oscillator will thus only change the evolution period of the atomic coherence. When the system has dissipation and a non-resonant driving field, the outfield and the vibrating status of the mechanical oscillator will not slow down the atom decoherence. Therefore, we can find some important physics phenomena via the differences and connections between the roles which mechanical oscillator and dissipation plays.In Chapter 7, the applications of Atom-cavity system was introduced in the field of precision measurement represented by quantum measurement.Finally, summary and outlook on the research are given.