doi:10.1038/nindia.2018.29 Published online 12 March 2018

Conflict between quantum, classical mechanics deepens

K. S. Jayaraman

An international team of researchers have reported1 new results that they say will have deep implications in understanding "non-locality" issues in quantum mechanics.

“Quantum and classical worlds are in sharp contrast in the way they behave," Arun Kumar Pati, Professor in the Quantum Information and Computation Group at the Harish-Chandra Research Institute in Allahabad and a co-author in the study, told Nature India.

Albert Einstein believed that quantum theory should be local and that the physical properties of a quantum system should be real. He, along with Boris Podolsky and Nathan Rosen, published a milestone paper2 concerning local realism predictions that go against quantum mechanics.

Despite Einstein's claims, Irish physicist John Bell in 1964 showed3 that non-locality is an inherent feature of the quantum world that could not be explained by any theory that preserved “locality”. He proved the contradiction between quantum mechanics and local realism, thus giving birth to the notion of "non-locality".

Quantum non-locality is a consequence of the ability of quantum systems to exist in "entangled" states, in which individual particles – no matter how far apart – are seemingly connected as if by magic. If a measurement is performed on one of the “entangled” particles, the state of the other is instantaneously modified – a behaviour that may have applications in quantum communications and teleportation. The notion of non-locality was further strengthened by Lucien Hardy, who reduced the non-locality proof to two-particle states by presenting the famous Hardy’s paradox4, confirming that quantum theory – to be consistent with the experiments – must be non-local.

The physicists Jing-Ling Chen (left) and Arun Kumar Pati

"We prove a new result in quantum mechanics by presenting the most general framework for the multi-particle Hardy’s paradox," says Pati. The generalised Hardy’s paradox includes previously known results as special cases and gives sharper conflicts between quantum and classical theories, says co-author Jing-Ling Chen at Nankai University of China. "In actual experiments, our method can display higher success probability in bringing out quantum non-locality and hence is stronger than the standard Hardy’s paradox."

These results not only advance the study of Bell's non-locality by presenting the most general framework for the Hardy's paradox, "but also provide a feasible proposal to experimentally observe the stronger paradox", the report says.

Pati says the primary outcome of the study is that the quantum world cannot be understood using classical notions. "Although over a hundred years have passed since the birth of quantum mechanics, it is surprising that one can still discover bizarre paradoxes. The conflicts between quantum and classical predictions continue changing the way we understand what quantum theory really means."


1. Jiang, S.-H. et al. Generalized Hardy's Paradox. Phys. Rev. Lett. 120, 050403 (2018) doi: 10.1103/PhysRevLett.120.050403

2. Einstein, A. et al. Can quantum-mechanical description of physical reality be considered complete? Phys. Rev. 47, 777 (1935) doi: 10.1103/PhysRev.47.777

3. Bell, J. S. On the Einstein Podolsky Rosen paradox. Phys. 1, 195-290 (1964) Article

4. Hardy, L. et al. Quantum mechanics, local realistic theories, and Lorentz-invariant realistic theories. Phys. Rev. Lett. 68, 2981 (1992) doi: 10.1103/PhysRevLett.68.2981

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