We present resonant four-wave mixing (FWM) signals involving a Rydberg state in a thermal Rb vapor. The
dynamic behavior of the FWM signals exhibits revival peaks shortly after the incident pulse, which are due to
coherent collective emission among all Doppler classes. The FWM signals have dephasing times up to 7 ns,
and strongly depend on the excitation bandwidth to the Rydberg state. Our numerical simulations based on a
four-level model including the whole Doppler broadened ensemble can describe the data quite well.
A high-storage efficiency and long-live quantum memory for photons is an essential component for the information processing in long-distance quantum communication and optical quantum computation. We demonstrated a 78% storage efficiency (SE) of coherent light pulses with a cold atomic medium based on the effect of electromagnetically induced transparency (EIT). We also obtained a large fractional delay of 74 at 50% SE, which is the best record to date. The measured fidelity of the memory is better than 90%. The results suggest the EIT light-matter interface can be readily applied to single-photon quantum states. Our work greatly advances the technology of EIT-based quantum memory for the practical quantum information applications.