  We study linear and nonlinear optical properties of electromagnetically
induced transparency (EIT) medium interacting with two quantized laser fields
for adiabatic EIT case. We show that EIT medium exhibits normal dispersion.
Kerr and higher order nonlinear refractive-index coefficients are also
calculated in a completely analytical form. It is indicated that EIT medium
exhibits giant resonantly enhanced nonlinearities. We discuss the response of
the EIT medium to nonclassical light fields and find that the polarization
vanishes when the probe laser is initially in a nonclassical state of no
single-photon coherence.
