  The possibility to store optical information is important for classical and
quantum communication. Atoms or ions as well as color centers in crystals offer
suitable two-level systems for absorbing incoming photons. To obtain a reliable
transfer of coherence, strong enough light-matter interaction is required,
which may enforce use of ensembles of absorbers, but has the disadvantage of
unavoidable inhomogeneities leading to fast dephasing. This obstacle can be
overcome by echo techniques that allow recovery of the information as long as
the coherence is preserved. Albeit semiconductor quantum structures appear
appealing for information storage due to the large oscillator strength of
optical transitions, inhomogeneity typically is even more pronounced for them
and most importantly the optical coherence is limited to nanoseconds or
shorter. Here we show that by transferring the information to electron spins
the storage times for the optical coherence can be extended by orders of
magnitude up to the spin relaxation time. From the spin reservoir it can be
retrieved on purpose by inducing a stimulated photon echo. We demonstrate this
for an n-doped CdTe/(Cd,Mg)Te quantum well for which the storage time thereby
could be increased by more than three orders of magnitude from the
picosecond-range up to tens of nanoseconds.
