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# Observation of PT-symmetric quantum interference

## Abstract

A common wisdom in quantum mechanics is that the Hamiltonian has to be Hermitian in order to ensure a real eigenvalue spectrum. Yet, parity–time (PT)-symmetric Hamiltonians are sufficient for real eigenvalues and therefore constitute a complex extension of quantum mechanics beyond the constraints of Hermiticity. However, as only single-particle or classical wave physics has been exploited so far, an experimental demonstration of the true quantum nature of PT symmetry has been elusive. In our work, we demonstrate two-particle quantum interference in a PT-symmetric system. We employ integrated photonic waveguides to reveal that the quantum dynamics of indistinguishable photons shows strongly counterintuitive features. To substantiate our experimental data, we analytically solve the quantum master equation using Lie algebra methods. The ideas and results presented here pave the way for non-local PT-symmetric quantum mechanics as a novel building block for future quantum devices.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

We acknowledge funding from the Deutsche Forschungsgemeinschaft (grants SCHE 612/6-1, SZ 276/9-2, SZ 276/12-1, BL 574/13-1, SZ 276/20-1 and SZ 276/21-1) and the Alfried Krupp von Bohlen und Halbach Foundation. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 800942. We also thank C. Otto for preparing the high-quality fused-silica samples used in all experiments presented here.

## Author information

Authors

### Contributions

F.K. and A.S. developed the idea. L.T. worked out the theory. F.K. designed the samples and performed the experiments. F.K., L.T., M.H., A.S. and S.S. analysed and discussed the results. A.S. and S.S. supervised the project. All authors co-wrote the manuscript.

### Corresponding author

Correspondence to A. Szameit.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary notes and figures.

## Rights and permissions

Reprints and Permissions

Klauck, F., Teuber, L., Ornigotti, M. et al. Observation of PT-symmetric quantum interference. Nat. Photonics 13, 883–887 (2019). https://doi.org/10.1038/s41566-019-0517-0

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