How to realize a negative refractive index material at the atomic level in an optical frequency range
The theoretical mechanism for realizing the negative refractive index with {\it electromagnetically induced transparency} (EIT) is studied. It is shown that in a three-level dense atomic gas, there is a frequency band in which the EIT medium will exhibit simultaneously negative electric permittivity and magnetic permeability in the optical frequency range, and the atomic gas thus becomes a left-handed material. The expressions for the electric permittivity and the magnetic permeability for the probe frequency is presented. One of the remarkable features of the present novel scheme is such that the obtained EIT left-handed material is {\it isotropic} and may therefore have some potentially important applications in the development of new techniques in quantum optics.