October 21, 2015

expert reaction to results from the loophole-free Bell test

Publishing in the journal Nature, a group of scientists have described their work on quantum theory, reporting an instantaneous interaction between two electrons separated by a distance of 1.3 km.

Prof. Jim Al Khalili, University of Surrey, said:

“Late last year I made a documentary on BBC4 in which we repeated the Bell’s inequality experiment with entangled photons along these lines, showing Einstein was wrong. At the time, we were aware of these remaining loopholes, but for the purposes of the TV show I wasn’t too concerned with the subtleties. However, I would say this new result is not so surprising as it only confirms what 99.9% of quantum physicists have known all along: that Einstein was wrong about the incompleteness of quantum mechanics and his insistence on local realism. However, what it does do is force us to start to think seriously about what non-local realism might actually mean: how can two entangled and separated particles ‘communicate’ faster than light? Saying it’s all wrapped up in the maths is not good enough. And saying it’s fine because we can never use it ourselves to send information faster than lightis also not good enough – after all, quantum entangled particles really do seem to violate Einstein’s special theory of relativity (nothing travels fats than light).   So basically, still work to be done, in my view.”

Dr Anthony Lain, Research Fellow, University of Bristol, said:

“The seminal experiments by the teams of Clauser and Aspect in the 1970s and 1980s were enough to convince many physicists that quantum physics is non-local as the theory predicts. Since those days our confidence in the theories of quantum mechanics has been strong enough to launch the major international research efforts into the quantum technologies that they support.

“However, impressive as these experiments were, they did leave some wiggle room for sceptics of quantum non-locality, in the form of loopholes, which remained in many of the subsequent experiments. The two most important of these are to do with the low efficiency of detecting quantum particles, and the distance between the two stations that each measure one particle from the two-particle entangled states, which should be great enough to rule out classical communication.

“These loopholes have been closed separately in different experiments, but not closed simultaneously in a single experiment. Most physicists believe that Nature is not capricious enough to change her behaviour based on the type of experiment being performed, but  simultaneously closing the important loopholes remained an important, interesting and open question to address. Perhaps more than that, it remained a great experimental challenge for physics – and for brave physicists.

“Hensen et al. have performed an outstanding experiment, requiring an impressive level of expertise, simultaneously closing booth loopholes for the first time, and once again confirming that quantum mechanics is weird in a way that we could not imagine if the theory hadn’t predicted it.

“The techniques used in this experiment are highly relevant to the growing research effort in quantum technologies. In particular, they help pave the way for distributing entangled states and implementing wide area networks for quantum communication.”

Prof. Kai Bongs, University of Birmingham School of Physics and Astronomy, said:

“This is a brilliant demonstration of how different quantum phenomena are from classical experience, underpinning the expectation that quantum technology will open up unprecedented capabilities to improve the future.” said.

Prof. John Morton, Professor of Nanoelectronics and Nanophotonics, UCL, said:

“On the one hand, the result of this experiment is totally expected: if you took a conference hall full of quantum physicists and said “hands up who still believes in local realism”, chances are the answer would be nobody. Then why the excitement? I would say it comes down to two factors:

“First, it’s exciting because despite the reluctant acceptance by scientists that quantum physics really does seem to produce what Einstein called “spooky action at a distant”, it remains profoundly unintuitive and throws up challenging philosophical questions. Previous demonstrations of the Bell test have had significant “loop-holes”, which can be used to explain away the results without having to accept the existence of this “spooky action”, while this new demonstration by Hanson and coworkers, closes the most significant loop-holes present in previous tests.

“Second, performing this experiment required overcoming major practical challenges, and its success represents a milestone in mankind’s ability to control light and matter at the deepest (quantum) level and over long (km) distances.”

‘Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres’ by B. Hensen et al. published in Nature on Wednesday 21^st October 2015. 

Declared interests

Prof. John Morton: nothing to declare.

No other interests received.