During the storage time, information about the amplitude of the
probe ®eld is contained in the population amplitudes de®ning the
atomic dark states. Information about the mode vector of the probe
®eld is contained in the relative phase between different atoms in the
macroscopic sample. The use of cold atoms minimizes thermal
motion and the associated smearing of the relative phase during the
storage time. (We obtain storage times that are up to 50 times larger
than the time it takes an atom to travel one laser wavelength. As seen
from equation (1), the difference between the wavevectors of the
two laser ®elds determines the wavelength of the periodic phase
pattern imprinted on the medium, which is 105 times larger than the
individual laser wavelengths).
The regenerated probe pulse in Fig. 2b has the same shape as the
`normal' EIT pulse shown in Fig. 2a. Figure 2c shows a case where
the optical coherence is stored in the atomic medium for more than
800ms before it is read out by the coupling laser. Here the amplitude