Bell Theorem Generalized
An international team of researchers from USI, PI, ICFO, and USTC has generalized Bell theorem by showing that any hypothetical theory of physics involving bipartite ‘exotic’ sources in addition to Bell’s local hidden variables cannot explain quantum theory predictions. Thus, Quantum Nonlocality is Genuinely Tripartite.
Quantum correlations are obtained when multiple parties perform independent measurements on a shared quantum state. For instance, the scenario displayed in figure (a) depicts quantum correlations arising from Alice, Bob and Charlie each performing measurements on the quantum state Q.
Bell’s seminal theorem proves that certain correlations predicted by quantum theory resist explanation in terms of any local theory based on local hidden variable, also called shared randomness (for instance when the quantum state is replaced with several dices in (a)). But what about alternative explanations for quantum correlations, in terms of an exotic bipartite resources E of an undiscovered causal theory generalizing quantum theory and shared randomness, as in (b)?
In two theoretical studies published in Physical Review Letters and Physical Review A, Xavier Coiteux-Roy from USI Lugano, Elie Wolfe from PI Waterloo, led by ICFO researcher Marc-Olivier Renou, have shown that no such exotic causal theory can account for all quantum correlations, thus generalizing Bell's theorem.
They proposed a concrete experiment, depicted in figure a, which achieves correlations impossible to simulate with a scenario like that in figure b. This certifies the fact that Quantum Nonlocality is Genuinely Tripartite. They generalized this result, showing that Quantum Nonlocality is Genuinely N-partite, for any N.
These theoretical results triggered a race between different labs around the world to see who could perform the experiment successfully. The team constituted by researchers Huan Cao, Chao Zhang, Bi-Heng Liu, Yun-Feng Huang, Chuan-Feng Li, Guang-Can Guo from USTC Hefei together with original authors Coiteux-Roy, Wolfe, Renou and ICFO researcher Gaël Massé, was able to accomplish such experiment, study that was published in Physical Review Letters, and which graces the cover of PRL. In parallel, a second similar experimental work performed in Shenzhen was communicated the same day, and a third experimental work closing the so called locality loophole (which requires distance and synchronization between the experimental devices) was realized in the Hefei National Laboratory a month after, showing the importance that such theoretical results had.
Marc-Olivier Renou comments “all experiments are valid proofs under some hypothesis called loopholes, which are rather difficult to avoid because of technological limitations. Now, closing the locality loophole was a limited improvement, as it is very natural to assume that the lack of synchronization should not affect the experimental result. The most important limitation is the post-selection loophole, a very non-natural assumption that states that some detection events can simply be ignored. For the coming years, the challenge will be to find a way to avoid this assumption, either by adapting the theoretical protocol to make it experimentally feasible, or by technological improvements. In such pursuit, the resulting experiment would definitely be a wonderful celebration of the recent physics Nobel price, showing that Bell-like experiments can go way further than what we previously expected in constraining the potential alternatives theories to quantum theory”.