Einstein-Podolsky-Rosen experiment with two spatially separated Bose-Einstein condensates*

In 1935, Einstein, Podolsky, and Rosen (EPR) considered an hypothetical experiment, in which measurements are performed on two spatially separated quantum systems in an entangled state. The entanglement results in strong correlations between the measurement outcomes, making it possible to predict complementary properties of one system based on measurements of the other, with an uncertainty product below the Heisenberg bound. This “EPR paradox” revealed a conflict between quantum mechanics and a local realist description of the world. Since then experiment realisation of the EPR paradox has been realized with small quantum systems, while a demonstration with massive many-particle systems has so far remained elusive.

We will present an experiment demonstrating EPR paradox with two spatially separated many-particle systems. Our experiments are based on a pseudo spin-1/2 Bose Einstein condensate (BEC) of Rubidium-87 on an atom chip. Following the scenario considered by EPR, we first entangle about 1400 atoms in a single condensate by engineering the interatomic interactions. Subsequently, we split this entangled many-particle system into two halves and separate them spatially. Our technique allows us to individually address the collective spins of the two BECs, thereby realizing arbitrary spin measurements on the two systems. We observe strong correlations between the split systems, allowing us to infer measurement results of non-commuting spin observables in one system from measurements on the other, demonstrating the paradoxical situation envisioned by EPR.

Our results show that the conflict between quantum mechanics and local realism does not disappear as the system size is increased to over a thousand massive particles. Besides the interest in understanding fundamental aspects of entanglement of many-body systems, our experiments enable new investigations in quantum-enhanced metrology using spatially split probes.