New quantum paradox reveals contradiction between broad beliefs

Quantum physicists at Griffith University have uncovered a new paradox that says, when it comes to certain beliefs of nature about “what it should give.”

Quantum theory is practically perfect in predicting the behavior we observe when we perform experiments on small objects such as atoms. But applying quantum theory to scales much larger than atoms, especially to observers making the measurements, presents difficult conceptual problems.

In a paper published in Nature Physics, an international team led by Griffith University in Australia has sharpened these issues into a new paradox.

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

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

“The second assumption is that experimental institutions can be freely chosen, allowing us to perform random experiments. And the third assumption is that once such a free choice is made, its influence can not spread faster in the universe than light,” he said. hy.

“Each of these fundamental assumptions seems very plausible, 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 widespread beliefs must be wrong. Giving up on one of them has far-reaching implications for our understanding of the world. “

The team identified the paradox by analyzing a scenario with well-separated entangled quantum particles combined with a quantum “observer” – a quantum system that can be manipulated and measured from the outside, but can even make measurements on a quantum particle.

“Based on the three fundamental assumptions, we have mathematically determined certain limits on what experimental results are possible in this scenario. But quantum theory, when applied to observers, predicts results that exceed these limits. In fact, we already have a evidence-based principle experiment using scattered photons (particles of light), “said Drs. Nora Tischler, a senior experimental author. “And we found a violation just as quantum theory predicted.”

“But our ‘observer’ had a very small brain, so to speak. It just has two memory states, which are realized as two different paths for a photon. That’s why we call it a trial-or-principle experiment, not. a concluding demonstration that one of the three fundamental assumptions in our paradox must be wrong, “she said.

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

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

The Center for Quantum Dynamics Laboratory in which the experiment was conducted is also part of the Center for Quantum Computation and Communication Technology, an Australian Research Council Center of Excellence.

“It has long been recognized that quantum computers are revolutionizing our ability to solve difficult computer problems,” said Professor Wiseman.

“What we did not know until we started this research is that they can also help answer difficult philosophical problems – the nature of the physical world, the mental world, and their relationship.”