Does quantum mechanics accurately depict reality, or is it merely our flawed method of interpreting the peculiar characteristics of minuscule entities? A notable experiment aimed at addressing this inquiry has been conducted using quantum computers, yielding unexpectedly solid results. Quantum mechanics genuinely represents reality, at least in the context of small quantum systems. These findings could lead to the development of more efficient and dependable quantum devices.

Since the discovery of quantum mechanics over a hundred years ago, its uncertain and probabilistic traits have confounded scientists. For instance, take superposition. Are particles truly existing in multiple locations simultaneously, or do the calculations of their positions merely provide varying probabilities of their actual whereabouts? If it’s the latter, then there are hidden aspects of reality within quantum mechanics that may be restricting our certainty. These elusive aspects are termed “hidden variables,” and theories based on this premise are classified as hidden variable theories.

In the 1960s, physicist John Bell devised an experiment intended to disprove such theories. The Bell test explores quantum mechanics by evaluating the connections, or entanglement, between distant quantum particles. If these particles exhibit quantum qualities surpassing a certain threshold, indicating that their entanglement is nonlocal and spans any distance, hidden variable theories can be dismissed. The Bell test has since been performed on various quantum systems, consistently affirming the intrinsic nonlocality of the quantum realm.

In 2012, physicists Matthew Pusey, Jonathan Barrett, and Terry Rudolph developed a more comprehensive test (dubbed PBR in their honor) that enables researchers to differentiate between various interpretations of quantum systems. Among these are the ontic perspective, asserting that measurements of a quantum system and its wavefunction (a mathematical representation of a quantum state) correspond to reality. Conversely, the epistemological view suggests that this wavefunction is an illusion, concealing a richer reality beneath.

If we operate under the assumption that quantum systems possess no ulterior hidden features that impact the system beyond the wave function, the mathematics of PBR indicates we ought to comprehend phenomena ontically. This implies that quantum behavior is genuine, no matter how peculiar it appears. PBR tests function by comparing different quantum elements, such as qubits in a quantum computer, assessing how frequently they register consistent values for specific properties, like spin. If the epistemological perspective is accurate, the qubits will report identical values more often than quantum mechanics would suggest, implying that additional factors are at play.

Yang Songqinghao and his colleagues at the University of Cambridge have created a method to perform PBR tests on a functioning IBM Heron quantum computer. The findings reveal that if the number of qubits is minimal, it’s possible to assert that a quantum system is ontic. In essence, quantum mechanics appears to operate as anticipated, as consistently demonstrated by the Bell test.

Yang and his team executed this validation by evaluating the overall output from a pair or group of five qubits, such as a sequence of 1s and 0s, and determined the frequency at which this outcome aligned with predictions regarding the behavior of the quantum system, factoring in inherent errors.

“Currently, all quantum hardware is noisy and every operation introduces errors, so if we add this noise to the PBR threshold, what is the interpretation? [of our system]? ” remarks Yang. “We discovered that if we conduct the experiment on a small scale, we can fulfill the original PBR test and eliminate the epistemological interpretation.” The existence of hidden variables vanishes.

While they successfully demonstrated this for a limited number of qubits, they encountered difficulties replicating the same results for a larger set of qubits on a 156-qubit IBM machine. The error or noise present in the system becomes excessive, preventing researchers from distinguishing between the two scenarios in a PBR test.

This implies that the test cannot definitively determine whether the world is entirely quantum. At certain scales, the ontic view may dominate, yet at larger scales, the precise actions of quantum effects remain obscured.

Utilizing this test to validate the “quantum nature” of quantum computers could provide assurance that these machines not only function as intended but also enhance their potential for achieving quantum advantage: the capability to carry out tasks that would be impractically time-consuming for classical computers. “To obtain a quantum advantage, you must have quantum characteristics within your quantum computer. If not, you can discover a corresponding classical algorithm,” asserts team member Haom Yuan from Cambridge University.

“The concept of employing PBR as a benchmark for device efficacy is captivating,” he notes. Matthew Pusey PhD from York University, UK, one of the original PBR authors. However, Pusey remains uncertain about its implications for reality. “The primary purpose of conducting experiments rather than relying solely on theory is to ascertain whether quantum theory can be erroneous. Yet, if quantum theory is indeed flawed, what questions does that raise? The entire framework of ontic and epistemic states presupposes quantum theory.”

Understanding Reality To successfully conduct a PBR test, it’s essential to devise a method of performing the test without presuming that quantum theory is accurate. “A minority of individuals contend that quantum physics fundamentally fails at mesoscopic scales,” states Terry Rudolph, one of the PBR test’s founders from Imperial College London. “This experiment might not pertain to dismissing certain proposals, but let me be straightforward: I am uncertain! – Investigating fundamental aspects of quantum theory in progressively larger systems will always contribute to refining the search for alternative theories.”

reference: arXiv, Doi: arxiv.org/abs/2510.11213