In 1935, Einstein, Podolsky and Rosen described a thought experiment - the EPR paradox. According to Einstein, the solution to the paradox was that quantum mechanics offers an incomplete picture of our world, and there exists a set of properties which are hidden from our view. In 1964, John Bell proposed a method - the Bell theorem - that would test for the existence of "hidden variables".
Here at the ANU we are in the process of experimentally testing these fundamental tenets of quantum mechanics with matter waves for the first time. We use an excited state of helium (in a metastable excited state - He*) cooled into a Bose-Einstein condensate (BEC) as our matter wave source to generate entangled matter wave pairs. Single-atom-detection of the atom pairs allows us to formulate Bell inequalities - allowing us to test non-local realism with macroscopic matter waves.
The main outcome of the project will be a new fundamental knowledge of large-scale EPR-entanglement and quantum nonlocality with massive particles. First laboratory demonstrations of EPR-entangled photon states that violated the classical bounds demanded by a Bell inequality have been heralded as among the most important experiments of 20th century quantum physics. Such demonstrations, however, have been so far realised only for massless photons and pairs of massive particles, but never for large ensembles of massive particles. Generation of large-scale entanglement is important for understanding how quantum mechanics works on mesoscopic and macroscopic scales, and to determine the ultimate size of future quantum devices.