# $ψ$-epistemic interpretations of quantum theory have a measurement problem

Joshua B. Ruebeck1, Piers Lillystone1, and Joseph Emerson2,3

1Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
2Institute for Quantum Computing and Department of Applied Math, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

### Abstract

$\psi$-epistemic interpretations of quantum theory maintain that quantum states only represent incomplete information about the physical states of the world. A major motivation for this view is the promise to provide a reasonable account of state update under measurement by asserting that it is simply a natural feature of updating incomplete statistical information. Here we demonstrate that all known $\psi$-epistemic ontological models of quantum theory in dimension $d\geq3$, including those designed to evade the conclusion of the PBR theorem, cannot represent state update correctly. Conversely, interpretations for which the wavefunction is real evade such restrictions despite remaining subject to long-standing criticism regarding physical discontinuity, indeterminism and the ambiguity of the Heisenberg cut. This revives the possibility of a no-go theorem with no additional assumptions, and demonstrates that what is usually thought of as a strength of epistemic interpretations may in fact be a weakness.

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### Cited by

[1] Piers Lillystone and Joseph Emerson, "A Contextual $\psi$-Epistemic Model of the $n$-Qubit Stabilizer Formalism", arXiv:1904.04268.

[2] Iman Marvian, "Inaccessible information in probabilistic models of quantum systems, non-contextuality inequalities and noise thresholds for contextuality", arXiv:2003.05984.

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