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

Old Dominion University Research Foundation, Norfolk VA

Investigators

Abstract

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.