Electromagnetically induced left-handedness in a dense gas of three   level atoms
  We discuss how a three level system can be used to change the frequency dependent magnetic permeability of an atomic gas to be significantly different from one. We derive the conditions for such a scheme to be successful and briefly discuss the resulting macroscopic electrodynamics. We find that it may be possible to obtain left handed electrodynamics for an atomic gas using three atomic levels. 
M. O. Oktel, \"O. E. M\"ustecapl{\i}o\u{g}lu