  We study the propagation of a probe light in an ensemble of $\Lambda$-type
atoms, utilizing the dynamic symmetry as recently discovered when the atoms are
coupled to a classical control field and a quantum probe field {[Sun {\it et
al.,} Phys. Rev. Lett. {\bf 91}, 147903 (2003)]}. Under two-photon resonance,
we calculate the group velocity of the probe light with collective atomic
excitations. Our result gives the dependence of the group velocity on the
common one-photon detuning, and can be compared with the recent experiment (E.
E. Mikhailov, Y. V. Rostovtsev, and G. R. Welch, quant-ph/0309173).
