Research On Laser-assisted Vacuum QED Effect In Atoms

Since the mid-1980s,with the advent and continuous development of chirped pulse amplification(CPA)technology,the current peak intensity of laser field has reached 1022?1023 W/cm2.This has greatly expanded the research scope of the interaction between laser and matter,and promoted the advent of several emerging research orientations,such as laser acceleration,strong field Quantum Electrodynamics(QED),and so on.Among them,the strong field QED mainly focuses on the correction of some traditional QED effects in the strong laser field or the QED effects directly caused by strong field,such as Compton scattering,Mott scattering,and the generation of electronpositron pairs based on the Schwinger mechanism.Recently,the influence of the laser field on some vacuum-related QED effects has also inspired people's attention,e.g.the Lamb Shift and the annihilation of electron-positron pairs.These studies both play a vital role in verifying the basic theory of QED more accurately,and have important practical significance for some applications relating to precision measurement,such as optical clocks for the next generation definition of 'seconds'.This thesis mainly focuses on the study of laser-assisted vacuum QED effect in atoms.(1)The relativistic correction of the annihilation of the positronium atoms by the high-frequency intense laser field.Based on the Kramers-Henneberger(KH)transformation,the High-Frequency Floquet theory(HEFT)is applied to calculate the correction of the ground state wave function of the electron even element atom by the linearly polarized high-frequency and intense laser field under the conditions of non-relativity and relativity.And on this basis,we focus on the discussion of influence of the relativity correction on the annihilation of the positronium atoms.The study has found that under the non-relativistic framework,due to the transformation of the one-peak Coulomb potential to the double-peaks dressed Coulomb potential,that is,the KH potential,the probability of the overlapping of the electron and positron wave functions is reduced,so that when the laser field intensity reaches 1018 W/cm2,the lifetime of the positronium atoms can be increased by about three orders of magnitude.In order to take relativistic effect into consideration,we adopted the method of semi-classical orbit approximation to calculate the relativistic correction of the ground state wave function of the positronium atoms.Calculations show that with the effect of relativity,the annihilation of the positronium atom is further suppressed,and the lifetime is increased by 140%compared with the case without relativity correction.This " relativistic stability" is mainly derived from the“figure-8”shaped orbit correction of electrons under a strong field,which further reduces the probability of overlapping of the electron and the positron wave functions,thereby suppressing annihilation.(2)The influence of low-frequency weak laser field on the annihilation of the positronium atoms.The time-dependent perturbation theory is used to analyze the wave function correction of the positronium atom under the condition of the linearly polarized low-frequency weak external field,and it is applied to the calculation of the lifetime of positronium atom.It is found that with the affection of the electric dipole transition,the ground state wave function of the electron will be corrected by the first-order p orbital and the second-order s and d orbitals.The reconstructed wave function significantly reduces the probability of overlapping of the electron and positron clouds.With the condition of E=107 V/cm,?=5 × 1012 Hz,the lifetime of positronium atom will increase by 5 orders of magnitude.Since a weak infrared laser is adopted in this research,this theoretical result can be presented via experiments more easier than the stability of positrons and electrons pairs caused by a high-frequency strong field.(3)The correction of the Lamb Shift of atoms under an external laser field.Taking the clock transition energy level of the commonly used optical clock as an example,using a fully quantized atom-laser field interaction model,it is calculated that the dressed atoms energy shift corrections are caused by vacuum fluctuations based on the twolevels and rotating wave approximation.It has been found that the laser dressed Lamb Shift corrections can reach tens of mHz for strontium,ytterbium and cadmium atoms.Considering about the Stark Shift caused by the laser field in the optical clock is several hundred mHz,the contribution of the hyperpolarization rate and the nonlinear polarization rate caused by the multipolar transition is dozens of mHz,our research results have important reference value for the further improvement of the current optical clock accuracy and the measurement of absolute frequency of the optical clocks.