Retrieval of multiple spin waves from a weakly excited, metastable   atomic ensemble
  The emission of light from a multiply excited atomic ensemble is examined and it is shown how symmetric (spin-wave) and non-symmetric states of excitation radiate into spatially separate field modes. This observation has potential application to single photon generation and spin wave entanglement, since in the presence of atomic interactions it can result in isolated single photon emission into a phase-matched field mode. 
F. Bariani, T. A. B. Kennedy
quant-ph physics.atom-ph