Queensland quantum physicists have come upon a new paradox that challenges some long-held beliefs about nature.

Quantum theory is excellent at predicting the behaviour observed in experiments on tiny objects like atoms.

However, applying quantum theory at scales much larger than atoms, in particular to observers who make the measurements, raises difficult conceptual issues.

In a new paper, an international team led from Griffith University in Queensland has sharpened those issues into a new paradox.

They say that when trying to fit the square peg of quantum laws through the round hole of the macro-scale reality, “something’s got to give”.

“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 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.

“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.

“One of these widely held beliefs must be wrong,” Dr Cavalcanti says.

“Giving up any one of them has far-reaching consequences for our understanding of the world.”

To demonstrate the paradox, the experts propose 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.

“Based on the three fundamental assumptions, we have mathematically determined limits on what experimental results are possible in this scenario,” says Dr Nora Tischler, a senior experimental author.

“But quantum theory, when applied to observers, predicts results which violate these limits.

“In fact, we have already performed a proof-of-principle experiment using entangled photons [particles of light]… and we found a violation just as quantum theory predicted.

“But our ‘observer’ had a very small ‘brain’, so to speak.

“It has just two memory states, which are realised as two different paths for a photon. That’s why we call it a proof-of-principle experiment, not a conclusive demonstration that one of the three fundamental assumptions in our paradox must be wrong,” she said.

With the advance of supercomputers and the looming age of quantum computers, the paradox could be put to the test.

“For a more definitive implementation of the paradox, our dream experiment is one where the quantum observer is a human-level artificial intelligence program running on a massive quantum computer,” said Professor Howard Wiseman, the leader of the project and Director of Griffith’s Centre for Quantum Dynamics, where the theoretical and experimental teams are based.

“That would be a pretty convincing test of whether quantum theory fails for observers, or whether one of the three fundamental assumptions is false. But that’s probably decades away.”

More details are accessible here.