Single-Photon Superradiance in Cold Atoms
  The interaction of an ensemble of atoms with common vacuum modes may lead to an enhanced emission into these modes. This phenomenon, known as superradiance, highlights the coherent nature of spontaneous emission, resulting in macroscopic entangled states in mundane situations. The complexity of the typical observations of superradiance, however, masks its quantum nature, allowing alternative classical interpretations. Here we stress how this picture changed with the implementation ten years ago of a new process for single-photon generation from atomic ensembles. We present then the last piece of evidence for the superradiant nature of such process, reporting the observation of an accelerated emission of the photon with a rate that may be tuned by controllably changing the number of atoms in the ensemble. Such investigation paves the way to a new, bottom-up approach to the study of superradiance. 
Rafael A. de Oliveira, Milrian S. Mendes, Weliton S. Martins, Pablo L.   Saldanha, Jos\'e W. R. Tabosa, and Daniel Felinto
quant-ph physics.optics