Group velocity of a probe light in an ensemble of Λ atoms under two-photon resonance -: art. no. 051802

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 , Phys. Rev. Lett. 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 of E. E. Mikhailov, Y. V. Rostovtsev, and G. R. Welch, e-print quant-ph/0309173 .