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

量子光学和冷原子物理学的结合彻底改变了科学和技术发展的几个领域。 量子光学是研究光粒子、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的相互作用。 （原子可以被冷却到大约50皮开尔文的温度，即绝对零度以上1度的万亿分之一。 相比之下，外太空的冷近真空约为3开尔文，室温约为300开尔文。 这些科学领域的结合导致了原子钟的改进，磁场和电场传感器的发展，重力测量和旋转运动，仅举几例。 与此同时，开发了全新的实践和基础研究领域，包括材料和等离子体物理的量子模拟，以及量子信息学的动态研究领域。 本项目主要研究冷原子气体中单光子超辐射和亚辐射的实验和理论。 这些都是突现效应，它不是发生在单个原子本身，而是需要一组原子之间的合作相互作用。 在超辐射中，从冷原子气体中发射的光的速度比单个原子快得多。 光也是以一个狭窄的圆锥体发射的，其行为就像一束光子。 在亚辐射中，情况正好相反;光平均以非常慢的速度从各个方向射出，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频率（或波长）的偏移和发射光的颜色纯度的损失。 因此，这些效应可能对原子钟或其他精密传感器的性能产生负面影响。 因此，该项目的主要目标之一是彻底了解超辐射和亚辐射，这对于优化这些设备的操作至关重要。 另一方面，可以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，光子被吸收到超辐射态，并迅速转移到长寿命的亚辐射态。 在稍后的可控时间，原子气体可以切换回超辐射状态，其中存储的光子被重新发射到其原始模式。 该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，以及单光子量子存储器的可能应用。在该项目中，冷铷原子的致密和冷样品是光学制备和询问的窄带和近共振探测光束。 原子样品是具有大纵横比的椭圆形，并且通常沿长轴沿着被光学激发。 这种几何形状在理论上已被证明会导致前向散射光中的增强的集体兰姆频移。 探测光束被准备为用于时间分辨研究的时间短脉冲，并且对于频移测量而言，探测光束相当长且光谱较窄。 增强和快速的发射被观察到在近向前的方向，而超辐射和次辐射发射的研究在离轴配置。在最初的研究中，观察到单光子超辐射率的过程中发现，增加光学深度，合作过程的特点，线性增加。 的光谱位移的共振到较低的频率，与光学深度的线性关系，也被测量。 该项目目前和正在发展的方面包括研究（a）超辐射脉冲在稠密气体中的传播效应。（B）包括两体超辐射和亚辐射在内的少体集体效应;（c）非均匀展宽过程，如多普勒展宽和陷阱诱导光移对超辐射和亚辐射位形之间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.