What comes after quantum physics

Quantum Physics: What Is Really Real?

This is exactly where the discussion comes to a standstill. Which of the many interpretations of quantum mechanics, if any, is correct? This question is difficult to answer experimentally, because there are only subtle differences between the models: in order to be sustainable, they have to predict essentially the same quantum phenomena as the very successful Copenhagen Interpretation. Most of his 20-year career in quantum technology, says University of Queensland physicist Andrew White, "has been like climbing a giant slippery mountain - you just can't get a hold."

That changed in 2011 with a theorem about quantum measurements that seemed to exclude all psi-epistemic models. But as it turned out on closer inspection, there was a loophole for these theories here too. Nevertheless, the theorem inspired many physicists and made them think seriously about ways to end the controversy through experiments - experiments in which one wants to test the reality of the wave function. Maroney was already developing a test that should work in theory, and a little later he and other scientists also found ways to put the test into practice. Finally, last year, Fedrizzi, White, and other physicists carried out the experiment.

The idea behind the experiment can be illustrated with two stacks of playing cards. One only contains red cards, the other only aces. "Now you get one of these cards and should find out which pile it comes from," explains Martin Ringbauer, also from the University of Queensland. If it is a red ace, according to the physicist, "there is an overlap and you cannot tell where it came from". However, knowing the number of different playing cards in each deck can at least calculate how often such ambiguous situations will arise.

Clearly ambiguous

A similar problem also occurs in quantum systems. For example, a single measurement cannot always clearly determine how a photon is polarized. "In the everyday world, for example, you can easily differentiate between west and west-southwest, but in quantum systems that is not so easy," confirms White. According to the Copenhagen interpretation, the question of polarization makes no sense because there is no answer - or at least not until another measurement precisely determines this property. According to the psi-epistemic models, however, the question makes perfect sense; only the experimenters - just like the card players - have too little information after just one measurement to answer it. As in the case of the cards, it is possible to estimate how much ambiguity can be attributed to this ignorance. A comparison with the value possible in the context of the Copenhagen interpretation then provides important clues.

This is exactly the approach that Fedrizzi and his team took. In a photon beam, the group determined the polarization of the particles and other properties. The overlaps observed here cannot be explained by psi-epistemic models. Instead, the results support the opposite position: If there is an objective reality, the wave function is real. "It is really impressive that the team was able to tackle a fundamental question with what is actually a very simple experiment," comments physicist Andrea Alberti from the University of Bonn.

However, the finding is not yet absolutely clear: Because the detectors detected only about a fifth of the photons used in the experiment, the team had to assume that the remaining photons behaved in the same way. That is a daring thesis - and so the group is currently working on closing the loophole and obtaining a reliable result. Meanwhile, Maroney's team in Oxford is working with a group at the University of New South Wales in Australia on similar experiments with ions, which are easier to detect than photons. "We could have a waterproof version of this experiment within the next six months," says Maroney.