How \psi-epistemic models fail at explaining the indistinguishability of quantum states

TL;DR

This paper demonstrates that pistemic models struggle to explain quantum state indistinguishability, especially in higher dimensions, and provides experimental methods to test and constrain these models.

Contribution

It proves limitations of pistemic models in higher dimensions and offers explicit states and measurements for experimental bounds in dimensions 3 and 4.

Findings

pistemic models are arbitrarily ineffective in dimensions   4 for certain states

Explicit states and measurements enable experimental bounds on classical-quantum overlap ratios

Maximally pistemic models can be more effectively ruled out in dimensions 3 and 4

Abstract

We study the extent to which \psi-epistemic models for quantum measurement statistics---models where the quantum state does not have a real, ontic status---can explain the indistinguishability of nonorthogonal quantum states. This is done by comparing the overlap of any two quantum states with the overlap of the corresponding classical probability distributions over ontic states in a \psi-epistemic model. It is shown that in Hilbert spaces of dimension $d \geq 4$, the ratio between the classical and quantum overlaps in any \psi-epistemic model must be arbitrarily small for certain nonorthogonal states, suggesting that such models are arbitrarily bad at explaining the indistinguishability of quantum states. For dimensions $d$ = 3 and 4, we construct explicit states and measurements that can be used experimentally to put stringent bounds on the ratio of classical-to-quantum overlaps in \psi-epistemic models, allowing one in particular to rule out maximally \psi-epistemic models more efficiently than previously proposed.