Measurements on the reality of the wavefunction (1412.6213v2)

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.

Overview of "Measurements on the Reality of the Wavefunction"

The paper "Measurements on the Reality of the Wavefunction" investigates the ontological status of the quantum wavefunction, a topic that has provoked significant interest and controversy in quantum mechanics. The central question addressed is whether the wavefunction represents an objective reality or merely encapsulates an observer's knowledge about a quantum system. This is explored by experimentally examining the ability of $\psi$-epistemic models, which assert that the wavefunction reflects our partial knowledge rather than the true state of reality, to explain quantum phenomena, particularly the indistinguishability of non-orthogonal quantum states.

Methodology and Results

The authors apply constraints on $\psi$-epistemic models by utilizing a theoretical framework that evaluates how much these models can account for the inability to distinguish non-orthogonal quantum states. The experimental focus is on single photons in three and four-dimensional systems. These photons are precisely prepared and measured using high-dimension dual encoding, leveraging both polarization and path degrees of freedom.

The key experimental result shows that no $\psi$-epistemic interpretation can fully account for the indistinguishability of non-orthogonal states in dimensions larger than two. Quantitative results indicate substantial violations of derived inequalities intended for validating $\psi$-epistemic models. For instance, in the case of ten prepared ququart states, a significant violation of the inequality by over 250 standard deviations underscores the inadequacy of maximally $\psi$-epistemic models to match quantum mechanical predictions.

Implications

The experimental conclusions drawn from this paper are profound for the interpretation of quantum mechanics. They strongly suggest that the wavefunction is not merely epistemic but has real ontological significance, meaning it directly corresponds to physical reality. This perspective has implications for existing quantum interpretations, potentially challenging views that limit the wavefunction to a statistical tool or a mere representation of knowledge.

From a theoretical standpoint, the results imply that realist interpretations of quantum mechanics that lean on $\psi$-epistemic models might not suffice, leading to the necessity of considering more $\psi$-ontic views, where the wavefunction represents real quantum states. The experiment places stringent bounds on the extent of overlap between classical probability distributions associated with quantum states, limiting how they can explain quantum state indistinguishability.

Speculative Perspectives

The discussed findings open dialogues about alternative quantum interpretations, including theories involving retrocausality or frameworks abandoning objective reality in favor of alternatives like Quantum Bayesianism (QBism). Such approaches challenge the conventional ontological model framework, potentially suggesting new ways of understanding quantum mechanics' foundational aspects.

Conclusion

In conclusion, the paper presents compelling evidence against maximally $\psi$-epistemic models explaining quantum state indistinguishability, advocating for a realist interpretation where the wavefunction is treated as an actual element of reality. It enriches the discourse on the nature of quantum mechanics, inviting further exploration and refinement of prevailing interpretations, with implications extending into the conceptual foundations of quantum theory. This can drive future theoretical and experimental research, enhancing our comprehension of quantum phenomena and the fundamental nature of reality.