Controllable steep dispersion with gain in a four-level N-scheme with   four-wave mixing
  We present a theoretical analysis of the propagation of light pulses through a medium of four-level atoms, with two strong pump fields and a weak signal field in an N-scheme arrangement. We show that the generation of four-wave mixing has a profound effect on the signal field group velocity and absorption, allowing the signal field propagation to be tuned from superluminal to slow light regimes with amplification. 
Nathaniel B. Phillips, Irina Novikova, Eugeniy E. Mikhailov, Dmitry   Budker, Simon Rochester
physics.atom-ph