When it comes to space and time, modern physics defies our intuition in the most dramatic way. Einstein's relativity theory tells us that time and space are intimately related and that absolute time is an illusion. Quantum mechanics, however, is at rest, and its predictions are perhaps even more astonishing than those of relativity.
In a nutshell, quantum theory tells us that two entangled particles behave as a single physical object, no matter how far apart they are. If a measurement is performed on one of these particles, the state of its distant twin is instantaneously modified.
This effect leads to quantum nonlocality, the fact that the correlation between results of local measurements performed on these particles are so strong, that they could not have been obtained from any pair of classical systems, such as two computers. To cut a long story short, it is as if quantum particles live outside space-time – and experiments confirm this.
Understanding this phenomenon of quantum inseparability, arguably the most counter-intuitive feature of the theory, represents a major challenge of modern physics. A key point is that inseparability appears under various forms in quantum mechanics. Understanding precisely the relation between these various forms is a long-sought-after goal.
Writing in Physical Review Letters, Dr Tamas Vertesi from the Hungarian Academy of Sciences and Dr Nicolas Brunner from the University of Bristol make a significant step forward in this direction. They show that the weakest form of entanglement – so-called undistillable entanglement – can lead to quantum nonlocal correlations, the strongest form of inseparability in quantum mechanics. According to Professor Pawel Horodecki, a quantum theorist at the Gdansk University of Technology, “entanglement is almost ‘invisible’ in such systems, which makes it very surprising that they can exhibit nonlocality”.
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