In 1935, Einstein asked a profound question about our understanding of Nature: are objects only influenced by their nearby environment? Or could looking at one object sometimes instantaneously affect another far-away object, as predicted by quantum theory? Then in 1947, eight years before his death, Einstein wrote to a friend that he could not seriously believe in quantum mechanics because “physics should represent a reality in time and space, free from spooky actions at a distance.” He was referring to quantum entanglement.
Entanglement unites two objects in a shared quantum state, linking their properties in a way that transcends the separation between them. Now, after almost a century of controversies and inconclusive tests, physicists are celebrating what looks like the first definitive evidence that quantum entanglement works.
The team led by Delft professor Ronald Hanson used two diamonds containing tiny traps for electrons with a magnetic property called spin and measured all entangled pairs across 1.3 km separating two laboratories.
Lead scientist Professor Ronald Hanson said: ‘Things get really interesting when two electrons become entangled. ‘Both are then up and down at the same time, but when observed one will always be down and the other one up. ‘They are perfectly correlated, when you observe one, the other one will always be opposite. ‘That effect is instantaneous, even if the other electron is in a rocket at the other end of the galaxy.’
The first demonstration of their loophole-free Bell test, reported on October 21st in an article published through Nature, confirms that particles placed into entanglement, share a relationship that cannot be explained by the classical physical laws that govern everyday life.
What’s the Bell-Test?
Bell test experiments or Bell’s inequality experiments are designed to demonstrate the real world existence of certain theoretical consequences of the phenomenon of entanglement in quantum mechanics which could not possibly occur according to a classical picture of the world, characterised by the notion of local realism. Under local realism, correlations between outcomes of different measurements performed on separated physical systems have to satisfy certain constraints, called Bell inequalities. John Bell derived the first inequality of this kind in his paper “On the Einstein-Podolsky-Rosen Paradox”. Bell’s Theorem states that the predictions of quantum mechanics cannot be reproduced by any local hidden variable theory.
What are the loopholes?
Though the series of increasingly sophisticated Bell test experiments has convinced the physics community in general that local realism is untenable, it remains true that the outcome of every single experiment done so far that violates a Bell inequality can still theoretically be explained by local realism, by exploiting the detection loophole and/or the locality loophole. The locality (or communication) loophole means that since in actual practice the two detections are separated by a time-like interval, the first detection may influence the second by some kind of signal. To avoid this loophole, the experimenter has to ensure that particles travel far apart before being measured, and that the measurement process is rapid. More serious is the detection (or unfair sampling) loophole, because particles are not always detected in both wings of the experiment. It can be imagined that the complete set of particles would behave randomly, but instruments only detect a subsample showing quantum correlations, by letting detection be dependent on a combination of local hidden variables and detector setting. Experimenters have repeatedly stated that loophole-free tests can be expected in the near future. On the other hand, some researchers point out the logical possibility that quantum physics itself prevents a loophole-free test from ever being implemented.
This test used pairs of single electrons, to make sure that all the entangled pairs were measured, allowing the team to close the detection loophole. In addition, the 0.8 miles (1.3km) distance between detectors was too far to allow light to travel between them in the time it took to ask a question and get an answer. This closed the locality loophole.
‘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 Professor Kai Bongs, from Birmingham University.
“The experiment has closed two of the three major loopholes beautifully, but two out of three isn’t three,” Dr David Kaiser, Professor of the History of Science at the Massachusetts Institute of Technology thinks a weakness of the experiment is an electronic system used to add randomness to the measurement, may be predetermined in a method that was not detected in the experiment. To try to close a final loophole a team will try an experiment to ensure the independence of measurement detectors by capturing light from distant objects on different sides of the Milky Way galaxy in 2015, then capturing the light from quasars in 2017 and 2018.
Loophole-ree Bell inequality violation using electron spins separated by 1.3 kilometres
Proposal for a Loophole-Free Bell Test Using Homodyne Detection
“Bell’s theorem and the experiments: Increasing empirical support to local realism”, Studies In History and Philosophy of Modern Physics