Non-locality & Quantum Theory
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Locality means that an object is influenced directly only by its immediate surroundings and not by remotely located objects.
The fact that quantum mechanics is at odds with this principle is not new. Back in the 1930s, Einstein, Podolsky, Rosen, and Schrödinger, first spotted this striking aspect of quantum theory as they pointed out the possibility of quantum entanglement.
This led them to think that something was not quite right with quantum mechanics and that the theory had to be completed in a way that would restore locality.
But this hope was to fall apartyears later, when Bell proved that quantum mechanics is an inherently nonlocal theory that is simply incompatible with any physical theory in which the principle of locality holds.
Bell argued that in any local theory there are limits to how distant events are correlated. Mathematically, the correlation statistics must obey certain inequalities, now called Bell inequalities. Bell showed that two observers measuring states that are entangled would observe strong correlations between the results of their measurements: such correlations can be so strong that they violate Bell’s inequalities.
This phenomenon, called quantum nonlocality, has been repeatedly observed experimentally. For instance, in quantum optics experiments, it is now standard to create pairs of photons entangled in polarization. Then, each photon of the pair is sent (e.g., via an optical fiber) to a distant observer holding a polarization analyzer. The observed correlations between the results of both polarization measurements can lead to Bell-inequality violations, providing evidence that the physical world can be nonlocal.
Entanglement and nonlocality were first considered as two facets of the same physical effect, but they are now recognized as two different concepts, as research carried out in our group has demonstrated.
However, the relation between them is yet to be fully understood.
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