  Einstein-Podolsky-Rosen (EPR) entanglement introduced in 1935 deals with two
particles that are entangled in their positions and momenta. Here we report the
first experimental demonstration of EPR position-momentum entanglement of
narrowband photon pairs generated from cold atoms. By using two-photon quantum
ghost imaging and ghost interference, we demonstrate explicitly that the
narrowband photon pairs violate the separability criterion, confirming EPR
entanglement. We further demonstrate continuous variable EPR steering for
positions and momenta of the two photons. Our new source of EPR-entangled
narrowband photons is expected to play an essential role in
spatially-multiplexed quantum information processing, such as, storage of
quantum correlated images, quantum interface involving hyper-entangled photons,
etc.
