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# Measurements on the reality 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 in direct correspondence to reality? Recent no-go theorems argued that if there was any objective reality, then the wavefunction must be real. However, that conclusion relied on debatable assumptions. Here we follow a different approach without these assumptions and experimentally bound the degree to which knowledge interpretations can explain quantum phenomena. Using single photons, we find that no knowledge interpretation can fully explain the limited distinguishability of non-orthogonal quantum states in three and four dimensions. Assuming that a notion of objective reality exists, our results thus strengthen the view that the wavefunction should directly correspond to this reality.

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## Acknowledgements

We thank M. S. Leifer, J. Barrett, O. J. E. Maroney and R. Lal for insightful discussions and R. Muñoz for experimental assistance. This work was supported in part by the Centres for Engineered Quantum Systems (Grant No. CE110001013) and for Quantum Computation and Communication Technology (Grant No. CE110001027). A.G.W. acknowledges support from a University of Queensland Vice-Chancellor’s Senior Research Fellowship, C.B. from the ‘Retour Post-Doctorants’ program (ANR-13-PDOC-0026) of the French National Research Agency and a Marie Curie International Incoming Fellowship (PIIF-GA-2013-623456) of the European Commission, and C.B., E.G.C. and A.F. acknowledge support through Australian Research Council Discovery Early Career Researcher Awards (DE140100489, DE120100559 and DE130100240 respectively). This project was made possible through the support of a grant from Templeton World Charity Foundation, TWCF 0064/AB38. The opinions expressed in this publication are those of the author(s) and do not necessarily reflect the views of Templeton World Charity Foundation.

## Author information

Authors

### Contributions

A.F., A.G.W., C.B., E.C. and M.R. conceived the study. A.F., M.R. and B.D. designed the experiment. C.B. provided the lists of states and measurements to be used. M.R. and B.D. performed the experiment, collected and analysed the data. All authors contributed to writing the paper.

### Corresponding author

Correspondence to A. Fedrizzi.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

Supplementary Information (PDF 5919 kb)

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Ringbauer, M., Duffus, B., Branciard, C. et al. Measurements on the reality of the wavefunction. Nature Phys 11, 249–254 (2015). https://doi.org/10.1038/nphys3233

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