Einstein-Podolsky-Rosen Experiment with spatially separated entangled Bose Einstein Condensates

In 1935, Einstein, Podolsky, and Rosen (EPR) conceived a Gedankenexperiment which still challenges our understanding of reality and locality today. While this experiment has been realized with small quantum systems, a demonstration of the EPR paradox with macroscopic many-particle systems remains an important challenge. Such systems are particularly closely tied to the concept of local realism in our everyday experience, and may serve as probes for new physics at the quantum-classical transition. Here we report a mesoscopic EPR experiment with two spatially separated Bose-Einstein condensates, each containing about 700 Rubidium atoms. Entanglement between these condensates results in strong correlations of their collective (Pseudo-) spins, allowing us to demonstrate the EPR paradox. Our results represent the first observation of the EPR violation with spatially separated, massive many-particle systems. They show that the conflict between quantum mechanics and local realism does not disappear as the system size increases to more than a thousand massive particles. Furthermore, spin entanglement in conjunction with individual control of the two condensates can form an important resource for quantum metrology. Our experimental setup is particularly well tailored for quantum enhanced multiparameter sensing, which we hope to investigate further in the future.