Nonvolatile atomic memory in the spontaneous scattering of light from cold two-level atoms

The bunching of light resulting from the spontaneous emission of an atomic ensemble is a well known effect, shared with various sources of thermal fields. It is, on the other hand, also an integral part of recently developed quantum memories for various applications. Here we theoretically and experimentally investigate the statistical correlations in light spontaneously scattered from a cold ensemble of two level atoms. We demonstrate that the bunching of light in this situation is both long-lived and robust to various parameters of the excitation process. This memory comes from constructive interference of the emitted light for particular distributions of positions of the atoms, being washed out by their movement. We offer both a first-principles quantum theory for the process and a corresponding classical theory. We also introduce various methods to treat long-term fluctuations of the experimental data in order to be able to compare it to our simple theories. As a result, we obtain a detailed picture of such spontaneous scattering processes in cold atomic ensembles, which may result in better control of quantum memories heralded by these processes.