  We study the $\Lambda$-atoms ensemble based quantum memory for the storage of
the quantum information carried by a probe light field. Two atomic Rabi
transitions of the ensemble are coupled to the quantum probe field and
classical control field respectively with a same detuning. Making use of the
hidden symmetry analysis developed recently for the on-resonance EIT case (Sun,
Li, and Liu, Phys. Rev. Lett. 91, 147903 (2003)), we show that the dark states
and dark-state polaritons can still exist for the case of two-photon resonance
EIT. Starting from these dark states we construct a complete class of
eigen-states of the total system. A explicit form of the adiabatic condition is
also given in order to achieve the memory and retrieve of quantum information.
