Continuous generation of delayed light
  We use a Raman four-wave mixing process to read-out light from atomic coherence which is continuously written. The light is continuously generated after an effective delay, allowing the atomic coherence to evolve during the process. Contrary to slow-light delay, which depends on the medium optical depth, here the generation delay is determined solely by the intensive properties of the system, approaching the atomic coherence lifetime at the weak driving limit. The generated light is background free. We experimentally probe these properties utilizing spatial diffusion as an 'internal clock' for the atomic evolution time. 
Slava Smartsev, David Eger, Nir Davidson, Ofer Firstenberg
quant-ph physics.atom-ph