Cause-and-effect explanations like “catnip causes cats to be happy”, “jokes cause laughter” and “exciting research causes Physics World articles” are a useful way to organize knowledge about the world. The mathematics of cause-and-effect underpin everything from epidemiology to quantum physics. In the quantum world, however, the link between cause and effect is not so straightforward. An international team of physicists has now used quantum violations of classical causality to better understand the nature of cause-and-effect. In the process, the team uncovered quantum behaviour in a situation where standard methods indicate that the system ought to be classical – a result that could have applications in quantum cryptography.
In quantum physics, a result known as Bell’s theorem states that no theory that incorporates local “hidden” variables can ever reproduce the correlations between measurement outcomes that quantum mechanics predicts. A similar result occurs in the theory of causal inference, where quantum systems likewise defy the rules of classical causal reasoning. The idea behind the causal inference approach is that while a statistical correlation between two variables can arise due to a direct causal relationship between them, the correlation may also contain the contribution of a hidden common cause. In some cases, this hidden contribution can be quantified, and this can be used to show that quantum correlations do exist even when Bell’s theorem cannot be violated.
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