Is Quantum Nonlocality Visible at the Macroscopic Scale?

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Scientists from the University of Vienna and the Austrian Academy of Sciences have shown that it is possible to fully preserve the mathematical structure of quantum theory in the macroscopic limit.

One of the most fundamental features of quantum physics is Bell nonlocality: the fact that the predictions of quantum mechanics cannot be explained by any local (classical) theory. This has remarkable conceptual consequences and far-reaching applications in quantum information. However, in our everyday experience, macroscopic objects seem to behave according to the rules of classical physics, and the correlations we see are local. Is this really the case, or can we challenge this view?

In a recent paper in Physical Review Letters, scientists from the University of Vienna and the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences have shown that it is possible to fully preserve the mathematical structure of quantum theory in the macroscopic limit. This could lead to observations of quantum nonlocality at the macroscopic scale.

Our everyday experience tells us that macroscopic systems obey classical physics. It is therefore natural to expect that quantum mechanics must reproduce classical mechanics in the macroscopic limit. This is known as the correspondence principle, as established by Bohr in 1920.[1]

A simple argument to explain this transition from quantum mechanics to classical mechanics is the coarse-graining mechanism:[2] if measurements performed on macroscopic systems have limited resolution and cannot resolve individual microscopic particles, then the results behave classically. Such an argument, applied to (nonlocal) Bell correlations,[3] leads to the principle of macroscopic locality.[4] Similarly, temporal quantum correlations reduce to classical correlations (macroscopic realism)[2] and quantum contextuality reduces to macroscopic non-contextuality.[5]

It was strongly believed that the quantum-to-classical transition is universal, although a general proof was missing. To illustrate the point, let us take the example of quantum nonlocality. Suppose we have two distant observers, Alice and Bob, who want to measure the strength of the correlation between their local systems. We can imagine a typical situation where Alice measures her tiny quantum particle and Bob does the same with his and they combine their observational results to calculate the corresponding correlation.

Since their results are inherently random (as is always the case in quantum experiments), they must repeat the experiment a large number of times to… Brinkwire News Summary.

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