Researchers have used a quantum computer to perform a key test suggesting that quantum mechanics accurately describes reality at small scales. The experiment confirms the 'ontic' view, where the wave function represents true quantum states. However, noise limits insights at larger scales.
Quantum mechanics has puzzled physicists since its discovery over a century ago due to its probabilistic nature. A superposition, for instance, raises questions: does a particle truly occupy multiple places, or is the wave function merely a tool for calculating probabilities? Hidden variable theories propose underlying realities beyond quantum descriptions, but experiments like John Bell's in the 1960s have supported quantum non-locality, ruling out such ideas.
In 2012, physicists Matthew Pusey, Jonathan Barrett, and Terry Rudolph developed the PBR test to distinguish between interpretations of quantum systems. The ontic view holds that the wave function—the mathematical description of quantum states—reflects reality, while the epistemic view sees it as an approximation hiding deeper truths. The PBR test compares quantum elements, like qubits, to check if outcomes match predictions; higher overlaps than expected would support the epistemic side.
Songqinghao Yang at the University of Cambridge and colleagues applied this test on an IBM Heron quantum computer. For small groups of qubits—pairs or sets of five—they measured outputs like strings of 1s and 0s, accounting for noise. The results aligned with quantum predictions, confirming the ontic view. “Currently, all quantum hardware is noisy, and there are some errors on all operations, so if we add in this noise on top of the PBR threshold, then what would happen to our interpretation [of our system]?” says Yang. “It turns out that if you do the experiment on a small scale, then we can still satisfy the original PBR test and we can rule out the epistemic interpretation.”
Challenges arose with the 156-qubit machine, where errors obscured distinctions between views. This limits conclusions about quantum reality at larger scales. Verifying 'quantumness' via PBR could benchmark devices for quantum advantage, the ability to outperform classical computers. “If you want to have quantum advantage, you need to have quantumness inside your quantum computers, or else you can find an equivalent classic algorithm,” notes team member Haomu Yuan.
Matthew Pusey, a PBR originator now at the University of York, finds using it as a performance benchmark intriguing but questions its implications for reality, as it assumes quantum theory's validity. Terry Rudolph at Imperial College London adds that testing on larger systems narrows alternative theories, though this experiment may not rule out specific breakdowns at mesoscopic scales. The study appears in arXiv (DOI: arxiv.org/abs/2510.11213).