doi:10.1038/nindia.2017.13 Published online 31 January 2017

Physicists 'steer' out of quantum mystery

K. S. Jayaraman

Two seemingly independent particles – no matter how far apart – appear connected to each other as if by magic. Physicists describe this as ‘non-locality’, a remarkable aspect of the quantum world with significant applications in communications. A team of Chinese and Indian physicists has now proposed a novel approach to verify non-locality1.

Non-locality occurs due to the phenomenon of “entanglement” connecting individual particles that then behave as a single physical object. If a measurement is performed on one of the “entangled” particles, the state of the other is instantaneously modified. This strange behaviour that may hypothetically have applications even in teleportation was dismissed by Albert Einstein as “spooky action at a distance”. In 1935 he, together with Boris Podolsky and Nathan Rosen, developed the “EPR Paradox” as a way of showing that quantum theory was inconsistent with other known laws of physics2.

Despite Einstein's misgivings, Irish physicist John Bell, in 1964, showed that non-locality is an inherent feature of the quantum world since the results predicted by quantum mechanics could not be explained by any theory which preserved “locality”.

The physicists Jing-Ling Chen (left) and Arun Kumar Pati
But verifying Bell's non-locality has been a long-standing problem. “All experiments till date required additional assumptions resulting in loopholes,” Arun Kumar Pati, at the Harish-Chandra Research Institute in Allahabad and a co-author in the study, told Nature India. “We report a novel approach that is free of any additional assumptions and provides an easy means for experimental verification of quantum non-locality”.

Their novel approach exploits the fact that the quantum states showing "Bell's non-locality" form only a “subset” of quantum states that have the property of “steering” – a phenomenon where the person in location A can control the quantum state of a particle at a distant location B by performing measurements on his or her "entangled" partner. "In our work we have shown that ‘Bell nonlocal states’ can be constructed from some steerable states. This finding offers a distinctive way to detect Bell's nonlocality.”

Jing-Ling Chen of Nankai University in Tianjin, China, and lead author added, “We anticipate that our work will trigger further experimental developments in this direction and may stimulate new quantum protocols.”

Theoretical physicist Dipankar Home at the Bose Institute in Kolkata says that in recent years the study of ‘Quantum Steering’ has been of considerable interest. “The study suggests that starting from certain quantum states which exhibit steering, one can generate quantum states which have the property of non-locality in accordance with Bell’s theorem,” he told Nature India.


1. Chen, J.-L. et al. Bell’s nonlocality can be detected by the violation of Einstein-Podolsky-Rosen Steering Inequality”. Sci. Rep. 6, 39063 (2016) doi: 10.1038/srep39063 

2. Einstein, A. Can quantum-mechanical description of physical reality be considered complete?” Phys. Rev. 47, 777 (1935)