  Quantum information science attempts to exploit capabilities from the quantum
realm to accomplish tasks that are otherwise impossible in the classical domain
[1]. Although sufficient conditions have been formulated for the physical
resources required to achieve quantum computation and communication [2], there
is an evolving understanding of the power of quantum measurement combined with
conditional evolution for accomplishing diverse tasks in quantum information
science [3-5]. In this regard, a significant advance is the invention of a
protocol by Duan, Lukin, Cirac, and Zoller (DLCZ) [6] for the realization of
scalable long distance quantum communication and the distribution of
entanglement over quantum networks. Here, we report the first enabling step in
the realization of the protocol of DLCZ, namely the observation of quantum
correlations for photon pairs generated in the collective emission from an
atomic ensemble. The nonclassical character of the fields is evidenced by the
violation of a Cauchy-Schwarz inequality for the two fields (1,2). As compared
to prior investigations of nonclassical correlations for photon pairs produced
in atomic cascades [7] and in parametric down conversion [8], our experiment is
distinct in that the correlated (1,2) photons are separated by a programmable
time interval delta t, with delta t ~ 400 nsec in our initial experiments.
