Is the phenomenon called quantum entanglement really necessary for describing the physical world or is it possible for some post-quantum theory without confusion? In a new study about which phys.org writes, physicists have mathematically proved that any theory with a classical limit – when it can describe our observations of the classical world, referring to the classical theory under certain conditions – must involve entanglement. Therefore, despite the fact that entanglement is at variance with the classical understanding, it must be the inevitable and most important property not only of the quantum theory, but of any nonclassical theory that has not yet been developed.

Physicists in the person of Jonathan Richens of the Imperial College of London and University College London, John Selby of Imperial College London and the University of Oxford and Sabri Al-Safi from the University of Nottingham-Trent published an article stating that confusion is inevitable a feature of any nonclassical theory, in Physical Review Letters.

"Quantum theory has many strange features compared to classical theory," says Richens. "By tradition, we study how the classical world comes out of the quantum world, but here we decided to reverse this reasoning to see how the classical world forms the quantum world. So we showed that one of the most strange features of the latter, quantum entanglement, is the inevitable consequence of going beyond the framework of the classical theory, or perhaps even a consequence of our inability to abandon the classical theory, to leave it behind. "

Although the full proof is much more detailed, the basic idea is that any theory describing reality must behave like a classical theory in some limit. This requirement seems rather obvious, but, as physicists show, it imposes serious limitations on the structure of any non-classical theory.

Quantum theory satisfies this requirement of having a classical limit in the process of decoherence. When a quantum system interacts with the external environment, it loses its quantum coherence, connectivity, and everything that makes it quantum. Thus, the system becomes classical and behaves as expected in the classical theory.

Physicists have shown that any non-classical theory that restores a classical theory must contain entangled states. To prove this, they went from the opposite: suppose, such a theory has no entanglement. And then they showed that without confusion any theory that restores the classical theory must itself be classical – and this contradicts the original hypothesis that such a theory should be nonclassical. This result means that the assumption of no confusion in such a theory will be false, and therefore any theory of this type should have it.

This result can only be the beginning of many other related discoveries, since it opens the possibility that other physical features of the quantum theory can be reproduced simply by requiring the theory to have a classical limit. Physicists suggest that such features as information causality (cause-effect relationship), bit symmetry and macroscopic locality can be proved, thanks to this single requirement. These results also give a clearer idea of how any future nonclassical, post-quantum theory should look.

"My future goals are to see if Bell's nonlocality can also be extracted from the existence of the classical limit," says Richens. "It would be interesting if all theories replacing classical theory would violate local realism."

Local realism is a combination of the locality principle with a "realistic" assumption that all objects have "objectively existing" values of their parameters and characteristics for any possible measurements that can be made over these objects * before * these measurements are made. Einstein, being, apparently, a supporter of local realism, liked in this connection to say that the Moon does not disappear from the sky, even if no one observes it. The data of modern quantum mechanics, based on the conducted experiments, call into question the adequacy of the model of local realism to the "device" of reality.