Cavity Quantum Electrodynamics And Its Applications Based On Atomic And Molecular Ensembles

Since the introduction of quantum theory,our understanding of the microscopic world has undergone a revolution.Cavities play an indispensable role in the study of quantum theory and the application of quantum technology.They can generate intracavity optical modes with extremely small mode volumes and make multiple trips of photons in the cavity,those enhance the interaction between light and matter in the cavity.Cavity quantum electrodynamics provides a theoretical basis for studying the evolution of dynamics in the quantum system of cavities.It focuses on the interaction between the optical field and the matter inside the cavity and has been developed for the study of quantum information,precision measurement,and other applications.It provides theoretical guidance for research and applications in fields such as quantum information and precision measurement.Atoms,with their strong non-linearity and rich energy level structure,have been the main research subjects in the study of quantum effects.With the development of techniques of atomic capture and internal state manipulation,atoms have been used in a wide range of applications,including quantum communication,quantum computing,and quantum metrology.Compared to individual atoms,atomic ensemble has richer external and internal dynamics,which is reflected in the saturation absorption spectrum and electromagnetically induced transparency.The collective effect of coupling a weak optical field to a atomic ensemble can effectively enhance the strength of the coupling.This thesis focuses on the theme of atomic ensemble and cavity coupling.The research and applications of the atomic ensemble-cavity coupling system for spectroscopic measurements of narrow linewidth energy levels and for frequency conversion are presented.The main innovative works accomplished are as follows.1.A new mechanism leading to an asymmetric saturated absorption spectrum,the nonlinear Fano-like effect,is revealed.The mechanism is verified by analytical derivation,numerical simulation and experimental testing.Precision spectroscopy with narrow energy level linewidths is widely used in sensing,metrology,standard reference of optical clock frequencies,determination of fundamental physical constants,and other areas.The accuracy of the spectral measurements directly affects the application of precision spectroscopy.Recently,several studies have shown that the saturated absorption spectra of vibration-rotational transitions of hydrogen-deuterium(HD)molecular exhibit asymmetric spectral profiles,when the HD molecules interact with standing wave field in a cavity.Due to the altered spectral line shape and unknown effects,it is not possible to obtain accurate spectral information directly from the obtained asymmetric spectra,which limits further research related to the vibration-rotational transitions of HD molecules.In this work,we start from a theoretical model that neglects the interactions between atoms and use the master equation,ensemble averaged approach to calculate the saturated absorption spectra of atoms/molecules with narrow linewidth energy levels interacting with intra-cavity standing wave.We revealed the underlying mechanism leading to the asymmetric saturated absorption spectra,which is named the nonlinear Fano-like effect.In this work,we have verified the rationality of the mechanism through analytical derivation,numerical simulation and experimental tests,that will be helpful to solve the bottleneck problem of this asymmetric spectrum for further application in the field of precision measurement.2.The conditions for the optimal conversion efficiency of A-type atomic ensemble coupled to microwave and optical wave resonators were determined.This optimal conversion efficiency shows good robustness to inhomogeneous broadening.Systems based on atomic ensemble for microwave-optical frequency conversion are different from that of spectroscopic calculations,in which atoms are considered to be independent of each other.This is because there is a coherence between the excitation phases of the atoms in the system used for frequency conversion.In this case,we use the spin-wave approximation to represent a coherent atomic system with a single excitation.Based on this,We investigated the mechanism of microwave-optical frequency conversion in the system of A-type atomic ensemble coupling with cavity modes,using spin waves as the conversion medium,and find the conditions for achieving optimal conversion efficiency.We have calculated the optimal conversion efficiency for the hot rubidium atoms and solid-doped systems,both with inhomogeneous broadening.It is found that the solid-doped system achieves a higher conversion efficiency by virtue of a higher number of atoms.The optimal conversion efficiency in this scheme is insensitive to inhomogeneous broadening,only the microwave coupling is a weak magnetic coupling,and the quality of the microwave and optical cavities is not high enough to limit further improvements.3.Under ideal conditions for light-wave coupling,the optimal conversion efficiency values for microwave-to-opticalwave frequency conversion based on the Riedberg atomic ensemble is only related to the degree of focusing of the microwave field.In order to achieve high conversion efficiency and to address the effect of weak coupling between atoms and microwave field on the conversion efficiency,we investigated the optimal conversion efficiency achievable with Riedelberg atoms.The microwave-coupled atomic system is treated as a superatom and the optical-coupled atomic system is still treated as a spin wave.The optimal conversion efficiency can be achieved when the critical coupling condition is met.In the ideal case(where the light and microwave fields are perfectly matched in mode),the optimal conversion efficiency achievable with Riedberg atoms coupling system is only related to the beam size of the microwave field being focused.When the radius of the beam waist is one microwave wavelength,a conversion efficiency of 3.8%can be achieved.In order to break the limitation on conversion efficiency imposed by the limited focusing capability of the lens,a microwave cavity can be used to further improve conversion efficiency.The study of cavity quantum electrodynamics based on ensemble is much more than that.In future work,we will still use a research approach combining theoretical research and applications.On the one hand,external properties of atoms,such as their momentum,can be considered to develop richer theoretical models.On the other hand,applications of cavity-based quantum electrodynamics theory based on atomic ensemble will be extended from spectral line shape and conversion efficiency analysis to aspects such as quantum channel and parameter estimation.