  An elementary quantum network operation involves storing a qubit state in an
atomic quantum memory node, and then retrieving and transporting the
information through a single photon excitation to a remote quantum memory node
for further storage or analysis. Implementations of quantum network operations
are thus conditioned on the ability to realize such matter-to-light and/or
light-to-matter quantum state mappings. Here, we report generation,
transmission, storage and retrieval of single quanta using two remote atomic
ensembles. A single photon is generated from a cold atomic ensemble at Site A
via the protocol of Duan, Lukin, Cirac, and Zoller (DLCZ) [Nature v.414, 413
(2001)] and is directed to Site B through a 100 meter long optical fiber. The
photon is converted into a single collective excitation via the dark-state
polariton approach of Fleischhauer and Lukin [Phys. Rev. Lett. v.84, 5094
(2000)]. After a programmable storage time the atomic excitation is converted
back into a single photon. This is demonstrated experimentally, for a storage
time of 500 nanoseconds, by measurement of an anticorrelation parameter a.
Storage times exceeding ten microseconds are observed by intensity
cross-correlation measurements. The length of the storage period is two orders
of magnitude longer than the time to achieve conversion between photonic and
atomic quanta. The controlled transfer of single quanta between remote quantum
memories constitutes an important step towards distributed quantum networks.
