Einstein-Podolsky-Rosen Correlations of Ultracold Atomic Gases

Einstein, Podolsky & Rosen (EPR) pointed out that correlations induced between quantum objects will persist after these objects have ceased to interact. Consequently, their joint continuous variables (CV), e.g., the difference of their positions and the sum of their momenta, may be specified, regardless of their distance, with arbitrary precision. EPR correlations give rise to two fundamental notions: {\em nonlocal ``steering''} of the quantum state of one object by measuring the other, and inseparability ({\em entanglement}) of their quantum states. EPR entanglement is a resource of quantum information (QI) and CV teleportation of light and matter waves. It has lately been demonstrated for {\em collective} CV of distant thermal-gas clouds, correlated by interaction with a common field. Here we demonstrate that collective CV of two species of trapped ultracold bosonic gases can be EPR-correlated (entangled) via {\em inherent} interactions between the species. This paves the way to further QI applications of such systems, which are atomic analogs of coupled superconducting Josephson Junctions (JJ). A precursor of this study has been the observation of quantum correlations (squeezing) in a single bosonic JJ.