New quantum paradox reveals contradiction between widely-held beliefs

Full article issued by Griffith University.

ARC-supported quantum physicists at Griffith University have conducted an experiment to unveil a new paradox that reveals the incompatibility of three commonly held beliefs about the nature of the world. 

Quantum theory is practically perfect at predicting the behaviour we observe when we perform experiments on tiny objects like atoms. But applying quantum theory at scales much larger than atoms, in particular to observers who make the measurements, raises difficult conceptual issues.

“The paradox means that if quantum theory works to describe observers, scientists would have to give up one of three cherished assumptions about the world,” said ARC Future Fellow Associate Professor Eric Cavalcanti, a senior theory author on the paper. 

“The first assumption is that when a measurement is made, the observed outcome is a real, single event in the world. This assumption rules out, for example, the idea that the universe can split, with different outcomes being observed in different parallel universes.”

“The second assumption is that experimental settings can be freely chosen, allowing us to perform randomised trials. And the third assumption is that once such a free choice is made, its influence cannot spread out into the universe faster than light,” he said.

“Each of these fundamental assumptions seems entirely reasonable, and is widely believed. However, it is also widely believed that quantum experiments can be scaled up to larger systems, even to the level of observers. But we show that one of these widely held beliefs must be wrong! Giving up any one of them has far-reaching consequences for our understanding of the world.”

The team established the paradox by analysing a scenario with well-separated entangled quantum particles combined with a quantum ‘observer’ – a quantum system which can be manipulated and measured from the outside, but which can itself make measurements on a quantum particle. The Centre for Quantum Dynamics laboratory in which the experiment was performed is also part of the ARC Centre of Excellence for Quantum Computation and Communication Technology.

 

Probing the reality of observations made by an artificial quantum intelligence. Artwork by Anthony Dunnigan.