Quantum computers reveal that the wave function is a real thing

Does quantum mechanics really reflect nature in its true form, or is it just our imprecise way of describing the strange properties of very small things? A famous test that may help answer this question has now been tried on a quantum computer, and it leads to a surprisingly specific conclusion. Quantum mechanics does indeed fully describe reality, at least for tiny quantum devices—and the results may help us build better, more reliable quantum machines.

Since quantum mechanics was first discovered more than a century ago, its uncertain and probabilistic nature has troubled physicists. Take superposition, for example: does a particle actually live in many places at once, or does calculating its position give us a range of probabilities about where it actually is? In the latter case, there may be some feature of reality hidden from quantum mechanics that limits our confidence. Such a feature would be a “hidden variable”, and so theories based on this idea are called hidden variable theories.

In the 1960s, physicist John Bell came up with an experiment rule out such theories. The Bell test examines quantumness by measuring how connected or entangled distant pairs of quantum particles are. If their quantum properties are maintained above a certain threshold—if their entanglement is what we call nonlocal and spans any distance—then we could rule out hidden variable theories. Since then, the Bell tests have been tried on many quantum systems, and they unanimously rule in favor of the inherent nonlocality of the quantum world.

In 2012, physicists Matthew Pusey, Jonathan Barrett and Terry Rudolph came up with an even more exploratory test (named PBR in their honor) that would allow experimentalists to distinguish between different interpretations of a quantum system. These include the ontic view that our measurements of a quantum system and its wave function—the mathematical description of its quantum states—represent reality. Another interpretation, called the epistemic view, argues that this wave function is a mirage and there is a deeper, richer reality beneath it.

If you believe that quantum systems don't have any other secret features that can influence the systems other than the wave function, then the mathematics of PBR shows that you should always have an ontic view of things – no matter how strange they look, quantum behavior is real. The PBR test works by comparing different quantum elements, such as a qubit inside a quantum computer, and measuring how often they read the same value for some property, such as their spin. If the epistemic view were correct, the number of times your qubits read the same value would be higher than quantum mechanics predicts, indicating that something else is going on underneath them.

Songqinghao Yang from the University of Cambridge and his colleagues developed a way to run the PBR test on a running IBM Heron quantum computer and saw that for a small number of qubits we can indeed say that quantum systems are ontic. That is, quantum mechanics appears to work as we thought, as the Bell tests have repeatedly shown.

Yang and his team conducted this test by measuring the overall output of pairs or groups of five qubits, such as strings of ones and zeros, and calculated how often that output matched their prediction of how a quantum system should behave, taking into account natural errors in the system.

“Currently, all quantum equipment is noisy and there are some errors in all operations, so if we add this noise on top of the PBR threshold, what happens to our interpretation? [of our system]? says Ian. “It turns out that if you do the experiment on a small scale, we can still satisfy the original PBR test and rule out the epistemic interpretation.” Hidden variables, away.

Although they were able to demonstrate this for a small number of qubits, they had difficulty doing the same for a larger number of qubits on IBM's 156-qubit machine. Noise or errors in the system became too great for researchers to distinguish between the two scenarios in the PBR test.

This means that the test cannot tell us whether the world is completely quantum. Perhaps at some scales the ontic view wins out, while at larger scales we cannot see exactly what quantum effects are doing.

Being able to test the “quantumness” of a quantum computer using this test could be a way to confirm that these devices do what we think they do, while also increasing the likelihood that they can demonstrate a quantum advantage – the ability to perform a task that would take an unreasonable amount of time for a classical computer. “If you want to have a quantum advantage, you need quantumness inside your quantum computers, otherwise you might find an equivalent classical algorithm,” says a team member. Haomu Yuan at Cambridge University.

“The idea of ​​using PBR as a benchmark for device performance is intriguing,” says Matthew Pusey at the University of York, UK, one of the first authors of PBR. But Pusey is less sure that this tells us anything about reality. “The main reason to do an experiment rather than rely on a theory is that you think quantum theory might be wrong. But if quantum theory is wrong, what question are you even asking? The whole setup of ontic and epistemic states presupposes quantum theory.”

To truly find a way to do a PBR test that tells us about reality, you will need to find a way to do it without assuming that quantum theory is true. “There is a minority of people who believe that quantum physics will fundamentally fail at some mesoscopic scale,” says Terry Rudolph at Imperial College London, another of the creators of the PBR test. “Although this experiment is unlikely to have anything to do with ruling out any particular such proposal – to be clear, I don't know one way or the other! – testing the fundamental features of quantum theory on ever larger systems always helps us narrow down the search space for alternative theories.”

Link: arXiv, DOI: arxiv.org/abs/2510.11213