Single-Photon Subradiance, Superradiance, and Emergent Cooperativity in Cold Atomic Matter

量子光学和冷原子物理学的融合使科学技术发展的几个领域发生了革命性的变化。量子光学是研究光、光子及其与物质相互作用的领域。冷原子物理学是研究冷原子及其与光和其他物质相互作用的学科。(原子可以冷却到大约50皮开尔文的温度，比绝对零度高出一万亿分之一度。相比之下，外层空间的近真空温度约为3开尔文，室温约为300开尔文。)这些加入的科学领域导致了原子钟的改进，磁场和电场传感器的发展，引力的测量，以及旋转运动，仅举几例。与此同时，在实际和基础研究方面也出现了全新的领域，包括材料的量子模拟和等离子体物理，以及量子信息学的动态研究领域。本项目致力于冷原子气体中单光子超辐射和亚辐射的实验和理论研究。这些都是紧急效应，单个原子本身不会产生这种效应，但需要一组原子之间的协同作用。在超强辐射下，冷原子气体发出的光的速度比单个原子快得多。这种光也是在一个狭窄的圆锥体中发射的，其行为类似于光子的喷射。在亚辐射中，反之亦然；光以非常慢的速度从各个方向平均射出，甚至可以在气体中储存很长一段时间。这些基本的量子光学过程伴随着发射光的频率(或波长)的移动和颜色纯度的损失。因此，这些效应可能会对原子钟或其他精密传感器的性能产生负面影响。因此，彻底了解超辐射和亚辐射，这是该项目的主要目标之一，对于优化这些设备的运行至关重要。另一方面，可以有利地利用这些效应来形成光的单光子存储器的基础。在这种情况下，光子在超辐射状态下被吸收，并迅速转移到长寿命的亚辐射配置。在以后的可控时间，原子气体可以切换回超辐射状态，在这种状态下，存储的光子被重新发射到其原始模式。这个项目的主要科学重点是研究和理解冷原子气体中单光子超辐射和亚辐射的现象学，并了解这些过程对量子传感器的影响，以及可能在单光子量子存储器中的应用。在该项目中，用光学方法制备了致密而寒冷的冷原子样品，并用窄带近共振探针束进行了探测。原子样品为大纵横比椭圆形，典型的沿长轴方向被光激发。从理论上讲，这种几何结构导致了前向散射光中集体Lamb频移的增强。对于时间分辨研究，探测光束被准备为时间上的短脉冲，而对于频移测量，探测光束相当长并且频谱更窄。在近前向观测到了增强的快速发射，而在离轴构型下研究了超辐射和亚辐射发射。在最初的研究中，观察到了单光子的超辐射；发现这一过程的速率随着光学厚度的增加而线性增加，这是合作过程的特征。还测量了共振到较低频率的光谱漂移，与光学厚度呈线性关系。该项目目前和正在发展的方面包括：(A)超辐射脉冲在稠密气体中的传播效应的研究。这些影响包括：(B)二体超、亚辐射的少体集体效应；(C)非均匀展宽过程对超、亚辐射组态的混合和受控耦合的影响。

The merger of quantum optics and cold atom physics has revolutionized several areas of scientific and technical development. Quantum optics is the field of study of particles of light, photons, and their interaction with matter. Cold atom physics is the investigation of cold atoms and their interaction with light and with other matter. (Atoms can be cooled to as low a temperature as about 50 picokelvin, a thrillionth of a degree above absolute zero. For comparison, the cold near vacuum of outer space is about 3 Kelvin, and room temperature is about 300 Kelvin.) These joined scientific areas have led to improved atomic clocks, development of sensors of magnetic and electric fields, measurements of gravitational forces, and rotational motion, to name a few. At the same time, entirely new areas of practical and fundamental investigation been developed, including quantum simulations of materials and plasma physics, and the dynamic research area of quantum informatics. This project is focused on experimental and theoretical study of single-photon super radiance and sub radiance in cold atomic gases. These are emergent effects, which do not occur for single atoms by themselves, but require cooperative interactions among a group of atoms. In super radiance, light emitted from a gas of cold atoms does so at a rate much faster than from a single atom. The light also is emitted in a narrow cone and behaves like a jet of photons. In sub radiance, the converse is true; light emerges on the average in all directions, at a very slow rate, and can even be stored for a long period of time in the gas. These fundamental quantum optical processes are accompanied by a shift of the frequency (or wavelength) and loss of the purity of the color of the emitted light. As such, these effects can have a negative influence on the performance of atomic clocks or other precision sensors. Thorough understanding of super and sub radiance, one of the main goals of this project, is thus essential to optimizing the operation of these devices. On the other hand, it is possible to use these effects to advantage to form the basis of a single photon memory for light. In this case, a photon is taken up in a super radiant state, and quickly transferred to a long lived sub radiant configuration. At a later controllable time the atomic gas can be switched back to a super radiant state, in which the stored photon is reemitted into its original mode. The main scientific focus of this project is to study and understand the phenomenology of single photon super and sub radiance in cold atomic gases, and to learn about the impact of these processes on quantum sensors, and possible applications to single photon quantum memories. In the project, a dense and cold sample of cold rubidium atoms is optically prepared and interrogated by a narrow-band and near-resonance probe beam. The atomic sample is elliptical with large aspect ratio, and is typically optically excited along the long axis. This geometry has been shown theoretically to lead to an enhanced collective Lamb frequency shift in the forward scattered light. The probe beam is prepared as a temporally short pulse for time resolved studies and is considerably longer and spectrally narrower for frequency shift measurements. Enhanced and rapid emission is observed in the near-forward direction, while both super radiant and sub radiant emission is studied in an off-axis configuration. In initial studies, single photon super radiance is observed; the rate for the process is found to increase linearly with increasing optical depth, characteristic of a cooperative process. A spectral shift of the resonance to lower frequency, with linear dependence on the optical depth, has also been measured. Current and developing aspects of the project include study of (a) superradiant pulse propagation effects in the dense gas. These can modify the linear cooperative scaling and limit the practical applications; (b) few-body collective effects including two-body super and sub radiance; (c) the influence of inhomogeneous broadening process, such as Doppler broadening and trap induced light shifts on mixing and controlled coupling between super and sub radiant configurations.