A key feature of quantum mechanics is the Heisenberg uncertainty principle, which states that complementary observables (such as position and momentum or two orthogonal components of a system’s spin) cannot be measured simultaneously with exact precision. In a famous 1935 paper, however, Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) noted that if a quantum system is entangled with another auxiliary system far away, then a measurement made on the latter can be used to infer properties of the former. This would lower the uncertainty for the system’s observables below the Heisenberg limit – and, they argued, create a paradoxical situation that clashes with classical notions of realism and locality.
Since 1935, EPR’s paradoxical thought experiment has been performed numerous times with small, few-body quantum systems. However, the question of whether the non-classical character of quantum mechanics can be observed in larger systems had remained open until recently, when researchers from the University of Basel, Switzerland performed an EPR experiment for the first time in many-body quantum systems. Their results, which they describe in Physical Review X, confirm that quantum spookiness survives even when the number of quantum particles exceeds 1000.
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