(a) Energy-level scheme for EIT experiments: two ground states (|g〉 and |s〉) are connected to one excited state (|e〉) by an optical transition. To reach the EIT condition, the frequency difference between the input and control beams must be equal to the splitting between |g〉 and |s〉. (c) Output pulse energy and storage efficiency as a function of the storage time, as measured by Heinze et al.  for three coherence times (T2) obtained under different sequences of radio frequency pulses. The inset shows a three-stripe image retrieved from the memory after different storage times.