Measurements on the reality of the wavefunction

TL;DR

This paper experimentally tests whether the quantum wavefunction is a real physical entity or just a representation of knowledge, providing evidence that supports the wavefunction's reality over knowledge-based interpretations.

Contribution

It offers the first experimental bounds on knowledge interpretations of the wavefunction in higher dimensions, strengthening the case for the wavefunction's reality.

Findings

Knowledge interpretations cannot fully explain quantum indistinguishability.

Results support the wavefunction as an element of reality.

Challenges to purely epistemic views of the wavefunction.

Abstract

Quantum mechanics is an outstandingly successful description of nature, underpinning fields from biology through chemistry to physics. At its heart is the quantum wavefunction, the central tool for describing quantum systems. Yet it is still unclear what the wavefunction actually is: does it merely represent our limited knowledge of a system, or is it an element of reality? Recent no-go theorems argued that if there was any underlying reality to start with, the wavefunction must be real. However, that conclusion relied on debatable assumptions, without which a partial knowledge interpretation can be maintained to some extent. A different approach is to impose bounds on the degree to which knowledge interpretations can explain quantum phenomena, such as why we cannot perfectly distinguish non-orthogonal quantum states. Here we experimentally test this approach with single photons. We find that no knowledge interpretation can fully explain the indistinguishability of non-orthogonal quantum states in three and four dimensions. Assuming that some underlying reality exists, our results strengthen the view that the entire wavefunction should be real. The only alternative is to adopt more unorthodox concepts such as backwards-in-time causation, or to completely abandon any notion of objective reality.