Quantum correlations do not imply instant causation

Research by an international team from Australia, the United Kingdom and Germany has shown that the gap between quantum phenomena and classical intuition is even bigger than previously thought.

In 2015, the universe was officially shown to be weird when a series of experiments demonstrated that entangled quantum particles remain instantly connected, no matter how far apart they are, through what Einstein famously dismissed as “spooky action at a distance”.

While it was shown that entanglement does not follow the classical rules of cause and effect, researchers continue to puzzle over how it really works.

Associate Professor Alessandro Fedrizzi from Heriot Watt University said that one popular explanation is that entangled objects could affect each other instantaneously, which ignores the universal speed-of-light limit.

He said, “In our experiment, we showed that this model cannot explain the experimental observations”.

We all intuitively understand and apply the concepts of cause and effect every day in our lives. Martin Ringbauer from the ARC Centre of Excellence for Engineered Quantum Systems said, “Picture yourself in a room where someone is flicking a light switch. Intuition and experience lets you establish a simple causal model: the switch causes the lights to turn on and off. In this case, correlation implies causation.”

“If we could entangle two lights, you would see them turn on and off at random, regardless of how far apart they are, with no obvious switch and in perfect lockstep. Einstein’s preferred explanation of this mysterious effect was that there must be a hidden light switch which acts as a common cause for our entangled lights.”

“In our experiment, our team set up individual photons to act like entangled lights and subjected them to two tests. In the first test, we essentially flicked the light switches ourselves to test a causal hypothesis. In the second, we tested a theory from one of our collaborators Rafael Chaves, which proposed that nonlocal causality cannot explain quantum entanglement.”

These results, published in the journal Science Advances, bring us a step closer to understanding the nature of this “spooky action at a distance”. Besides the fundamental importance, they also have potential applications for cyber-security. They can for example be used to increase the level of trust we have into quantum encryption devices.


Media: Martin Ringbauer (+61 7 3365 2444, m.ringbauer@uq.edu.au); Dr Alessandro Fedrizzi (+44 131 451 3649, A.Fedrizzi@hw.ac.uk); Tara Roberson, Communications, ARC Centre of Excellence for Engineered Quantum Systems (0404 516 635, t.roberson@uq.edu.au)

Access the paper here

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Major funding support

Australian Research Council