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VP(R)'s Picks: Quantum Mechanics Speeds Up Discovery of Cause & Effect

Vice-President (Research)'s Picks: Quantum Mechanics Speeds Up Discovery of Cause & Effect
(Photo credit: Croucher Foundation)


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The ability to identify cause-effect relations is an essential component of the scientific method. New research by Professor Giulio Chiribella greatly speeds up that process by using quantum strategies that open the path for applications ranging from the design of communication networks to the development of intelligent quantum machines.

The classical approach to identifying cause-effect relations is to formulate different hypotheses and test them against each other – for instance, to determine the effect of a drug, scientists administer it to some patients while others receive a placebo and the results are compared. But such techniques become inadequate when dealing with quantum phenomena, where responses to interventions can differ strikingly from those of classical random variables.

New research by Professor Chiribella and his student Daniel Ebler, published in Nature Communications, shows that quantum algorithms offer powerful advantages over all classical algorithms when it comes to discovering cause-effect relations. This is significant because classical algorithms are widely used in statistics and artificial intelligence and were the focus of the 2011 Turing Award conferred to Judea Pearl, who pioneered this field.

The key finding of Chiribella and Ebler is that because quantum particles can travel along many paths at the same time – a phenomenon called quantum superposition – they can be arranged to explore multiple experimental setups and test them in new ways, which reduces the probability of errors by an exponential amount compared with the best classical setups. This principle applies to a broad set of tasks, such as deciding whether there is a causal link between two given variables and identifying the cause of a given variable.

Professor G. Chiribella (left) and his student Daniel Ebler
Professor G. Chiribella (left) and his student Daniel Ebler

The ability to identify cause-effect relations much more quickly than before has broad potential for application, such as future quantum communication networks that speedily identify the presence of communication links, and the design of intelligent quantum machines.

Professor Chiribella has also done pioneering research on the ultimate precision limits of clocks and gyroscopes, the foundations of quantum mechanics, and the theory of quantum causal networks. He is the recipient of the 2010 Hermann Weyl Prize, a CIFAR-Azrieli Global Scholars Fellowship 2016, and a Croucher Senior Research Fellowship 2018-19. 

Testing causal hypotheses in the black box scenario.

(Image reproduced/adapted from Springer Nature under CC BY 4.0: Chiribella G. and Ebler D., “Quantum speedup in the identification of cause-effect relations”, Nature Communications, 2019, 10, 1472, 1-8.)