Study On Quantum Entanglement Induced By Virtual Photons In Ultrastrongly Coupled Systems

In recent years,people have strengthened research on the theory and experiment of ultrastrong-coupling systems,which greatly promoted the development of ultrastrong-coupling regime.When the strength of the interaction in the coupled physical system enters the ultrastrong-coupling regime,the conditions of the rotating-wave approximation are no longer met,and the counterrotating terms must be considered.With the deepening of people's research on the counterrotating terms in ultrastrong-coupling systems,more and more novel physical phenomena have been discovered.One of these most interesting phenomena related to the counterrotating terms in the ultrastrong-coupling regime is the generation of virtual excitations.Based on the ultrastrongly-coupled two-harmonic-oscillator and three-harmonicoscillator systems,we find that the counterrotating terms can induce the quantum entanglement between two harmonic oscillators and virtual excitations maintain this phenomenon.This paper will open a route to the study of quantum information processing and quantum physics based on virtual excitations.This thesis consists of the following chapters:In the first chapter,we firstly introduces the background knowledges such as the development history of the coupling regime between light and matter,and the current status and development of ultrastrong-coupling regime.Then we introduce the definition of quantum entanglement and the quantify of the degree of entanglement.In the second chapter,we study the effect of quantum entanglement maintained by virtual excitations in an ultrastrongly-coupled two-harmonicoscillator system.Here,the quantum entanglement is caused by the counterrotating interaction terms and hence it is maintained by the virtual excitations.We obtain the analytical expression for the ground state of the system and analyze the relationship between the average virtual excitation numbers and the ground-state entanglement.The quantum squeezing phenomenon in the ground state is analyzed by calculating the fluctuations of the rotated quadrature operators.We also study the entanglement dynamics between the two oscillators in both the closed-and open-system cases.In the latter case,the quantum master equation is microscopically derived in the normal-mode representation of the coupled-oscillator system.In the third chapter,we further study the quantum entanglement maintained by virtual excitations as a medium in the ultrastrongly coupled threeoscillator system.Correspondingly,we study the relationship between the ground state entanglement of any two harmonic oscillators and the average virtual excitation numbers.Then,we calculate the average virtual excitation numbers in the oscillators and the entanglement dynamics between any two oscillators in both closed-and open-systems cases.When the counterrotating terms exist,there is quantum entanglement between the harmonic oscillators,and this phenomenon is maintained by virtual excitations.In the fourth chapter,we summarize the main contents of this thesis,analyze the difficulties and challenges in the field of ultrastrong couplings,and provide a prospect for the research in this field.