  Light storage, the controlled and reversible mapping of photons onto
long-lived states of matter [1], enables memory capability in optical quantum
networks [2-6]. Prominent storage media are warm alkali gases due to their
strong optical coupling and long-lived spin states [7,8]. In a dense gas, the
random atomic collisions dominate the lifetime of the spin coherence, limiting
the storage time to a few milliseconds [9,10]. Here we present and
experimentally demonstrate a storage scheme that is insensitive to
spin-exchange collisions, thus enabling long storage times at high atomic
densities. This unique property is achieved by mapping the light field onto
spin orientation within a decoherence-free subspace of spin states. We report
on a record storage time of 1 second in cesium vapor, a 100-fold improvement
over existing storage schemes. Furthermore, our scheme lays the foundations for
hour-long quantum memories using rare-gas nuclear spins.
