Coherent backscattering of light by saturated atoms

本课题旨在对冷原子对强激光的相干后向散射（CBS）进行理论研究。重要的是要了解量子干涉是如何受到基本量子力学失相机制的影响的，量子干涉对这种现象的出现至关重要，如光子自旋翻转或非弹性散射。这对于光在冷原子气体中从弱到强的转变，或安德森定位，以及潜在的技术应用，如随机激光，具有重要的影响。先前的研究探索了潜在干涉效应对原子偶极跃迁简并、原子残余热运动或非弹性散射的依赖。关于后一种机制，许多基本问题仍未解决，例如高阶散射贡献如何受到非线性饱和效应的影响，或者CBS是否在光计数统计中留下可检测的痕迹。此外，尽管已有实验结果与之相匹配，但非弹性散射与其他两种失相机制（即简并和热运动）中的任何一种的联合影响迄今尚未在理论上得到考虑。为了解决这些问题，我们将开发一种原子-激光相互作用的非微扰描述，将基于少量原子的主方程的数值方法与用于处理许多原子无序样品的新分析工具相结合。回答这些问题将是迈向这个项目最终目标的重要一步——一个强激光在冷原子气体中多次散射的理论。

The present project aims at the theoretical study of coherent backscattering (CBS) of intense laser light by cold atoms. It is important to understand how quantum interference, crucial for the emergence of this phenomenon, can be affected by fundamental quantum mechanical dephasing mechanisms - such as photon spin flips or inelastic scattering. This has important consequences for the transition from weak to strong, or Anderson, localization, of light within cold atomic gases, as well as for potential technological applications, such as random lasers. Previous studies explored the dependence of the underlying interference effect on the degeneracy of the atomic dipole transitions, on residual thermal motion of the atoms, or on inelastic scattering. Concerning the latter mechanism, many fundamental questions are still unresolved, such as how higher-order scattering contributions are affected by nonlinear saturation effects, or whether CBS leaves a detectable trace in the photocount statistics. Furthermore, the combined impact of inelastic scattering together with either one of the two other dephasing mechanisms (i.e., degeneracy and thermal motion) has so far not been considered theoretically, although there are already experimental results to be matched with. To address these questions, we shall develop a non-perturbative description of the atoms-laser interaction, combining numerical approaches based on master equations for a small number of atoms with new analytical tools for treating disordered samples of many atoms. Answering them will be an important step towards the ultimate goal of this project - a theory for multiple scattering of intense laser light in cold atomic gases.