Things can be real, or certain, but not both

Experimenting within quantum theory is an extremely complex process, where common intuitions are regularly inverted within shifting reality. Over the years, several quantum features and methods of their study have been identified. An international group of scientists has investigated a new set of assumptions and proposed a novel experiment, to test the consequences of making quantum theory more intuitive.

“While quantum theory is the science behind almost all of our technology, its disconnect with our everyday intuitions is still worrisome and actively researched,” said lead author Associate Professor Daniel Terno from Macquarie University.

“How do you find your way in a reality which is shifting, where the opposites are allowed to co-exist? Moreover, how do you conduct experiments in it? These are the questions that must be answered when dealing with the floating world of quantum mechanics.”

Throughout the development of quantum theory, a set of reasonable ideas has led to strange paradoxes, such as Schrödinger’s cat, which is both dead and alive. Another result of quantum mechanics is that every object can behave as a particle or as a wave, given the right conditions.

Associate Professor Terno and colleagues proposed an experiment, using wave-particle duality as their starting point, to investigate a new and more comprehensible set of assumptions:

1. Every object at any time is really a particle or a wave, but not both (objectivity).
2. If you know enough you can predict everything (determinism).
3. Speed of light is the ultimate limit (locality).

In taking these assumptions and applying them to an experiment, where the measuring device is controlled by a Schrödinger’s cat-like state, the research team reached some perplexing paradoxes.

“Only after the cat was found to be dead or alive were we able to tell if what we did was to look for a particle or for a wave,” said Associate Professor Terno. “Then these three innocent-looking ideas result in predictions that would contradict an experiment. The universe simply does not work like that: you can see things to be real, or certain, but not both.”

Then the researchers tweaked their initial assumptions, replacing the third assumption with the requirement that how you set your detectors does not affect the system you study before they interact. This tweak led to another strange result: it is not only that our quantum world is not like that, but such a combination cannot be realised in any universe.

The experiment has since been realised by dozens of research groups worldwide and made complicated experiments much simpler, such as an experiment formerly requiring 40 m of optical cable now being performed on just a single chip. It has been published in the journal Nature Communications.

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