FIGURE 2. Measurements of delayed and revived probe pulses.

Open circles (fitted to the dotted gaussian curves) show reference pulses obtained as the average of 100 probe pulses recorded in the absence of atoms. Dashed curves and filled circles (fitted to the solid gaussian curves) show simultaneously measured intensities of coupling and probe pulses that have propagated under EIT conditions through a 339-m-long atom cloud cooled to 0.9 K. The measured probe intensities are normalized to the peak intensity of the reference pulses (typically, _p/ _c = 0.3 at the peak). a, Probe pulse delayed by 11.8 s. The arrow at 6.3 s indicates the time when the probe pulse is spatially compressed and contained completely within the atomic cloud. (The intersection of the back edge of the reference pulse and the front edge of the delayed pulse defines a moment when the tail of the probe pulse has just entered the cloud and the leading edge is just about to exit.) b, c, Revival of a probe pulse after the coupling field is turned off at t = 6.3 s and turned back on at t = 44.3 s and t = 839.3 s , respectively. During the time interval when the coupling laser is off, coherent information imprinted by the probe pulse, is stored in the atomic medium. Upon subsequent turn-on of the coupling field, the probe pulse is regenerated through coherent stimulation. The time constants for the probe and coupling PMT amplifiers are 0.3 s and 3 s, respectively. The actual turn on/off time for the coupling field is 1 s, as measured with a fast photodiode. d, Measured transmission of the probe pulse energy versus storage time. The solid line is a fit to the data, which gives a 1/e decay time of 0.9 ms for the atomic coherence.