Quantum computers reveal that the wave function is real

Does quantum mechanics really reflect nature in its truest form, or is it just our imprecise way of describing the strange properties of the very small? A famous test that can help answer this question has now been tested on a quantum computer, and it comes to a surprisingly concrete conclusion. Quantum mechanics really does completely describe reality, at least for small quantum devices – and the results could help us build better, more reliable quantum machines.

Since the discovery of quantum mechanics more than a century ago, its uncertain and probabilistic nature has troubled physicists. Take, for example, a superposition: does a particle actually inhabit multiple places at once, or does calculating its position give us a range of probabilities as to where it actually is? If so, there may be a feature of reality hidden from quantum mechanics that limits our certainty. Such a feature would be a “hidden variable”, and theories based on this idea are therefore called hidden variable theories.

In the 1960s, physicist John Bell designed an experiment to rule out such theories. A Bell test probes quantumism by measuring how closely distant pairs of quantum particles are related or entangled. If their quantum properties are maintained above a certain threshold – if their entanglement is what we call non-local, extending over any distance – then we could rule out theories of hidden variables. Since then, Bell tests have been attempted for numerous quantum systems, unanimously ruling in favor of the non-locality inherent in the quantum world.

In 2012, physicists Matthew Pusey, Jonathan Barrett, and Terry Rudolph proposed an even more thorough test (called PBR in their honor), which would allow experimentalists to differentiate between different interpretations of a quantum system. These include the ontic view, according to which 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, says that this wave function is a mirage and that there is a deeper, richer reality underneath.

Assuming you believe that quantum systems have no other secret features that can affect the systems beyond the wave function, then the mathematics of PBR shows that you should always have an ontic view of things – that, no matter how strange they may seem, quantum behaviors are real. The PBR test works by comparing different quantum elements, like a qubit in a quantum computer, and measuring how often they read the same value for certain properties, like 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 there is something else going on underneath.

Songqinghao Yang of the University of Cambridge and colleagues developed a way to perform the PBR test on a working IBM Heron quantum computer, and they found that for a small number of qubits, we can indeed say that quantum systems are ontic. In other words, quantum mechanics appears to work as we thought, just as Bell’s tests have repeatedly found.

Yang and his team performed this check by measuring the overall output produced by pairs or groups of five qubits, such as strings of 1s and 0s, and calculated how often this output matched their prediction of the behavior of a quantum system, taking into account the system’s natural errors.

“Currently, all quantum hardware is noisy, and there are errors on all operations, so if we add this noise above the PBR threshold, what would happen to our interpretation? [of our system]?” Yang said. “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, go away.

Although they were able to demonstrate this for a small number of qubits, they struggled to do the same for a larger number of qubits on the 156-qubit IBM machine. The noise, or errors, in the system became too great for researchers to distinguish between the two scenarios in a PBR test.

This means that the test cannot tell us whether the world is completely quantum. It could be that at some scales the ontic view wins out, while at larger scales we are not able to see precisely what quantum effects are doing.

Being able to check the “quantum character” of a quantum computer using this test could be a way to confirm that these devices do what we think they do, as well as making them more likely to display a quantum advantage – the ability to perform a task that would take a classical computer an unreasonable amount of time. “If you want to have a quantum advantage, you have to have quantum in your quantum computers, otherwise you can find an equivalent classical algorithm,” says team member Haomu Yuan from the University of Cambridge.

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

To really find a way to do a PBR test that would tell us about reality, you need to find a way to do it without presupposing that quantum theory is correct. “A minority of people believe that quantum physics will fundamentally break down at some mesoscopic scale,” says Terry Rudolph of Imperial College London, another of the initiators of the PBR test. “While this experiment is probably not relevant to ruling out such a specific proposal – to be clear, I don’t know one way or the other! – testing the fundamental features of quantum theory on ever-larger systems still helps us narrow the search space for alternative theories.”

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