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# Experimental Greenberger–Horne–Zeilinger entanglement beyond qubits

## Abstract

Quantum entanglement is important for emerging quantum technologies such as quantum computation and secure quantum networks. To boost these technologies, a race is currently ongoing to increase the number of particles in multiparticle entangled states, such as Greenberger–Horne–Zeilinger (GHZ) states. An alternative route is to increase the number of entangled quantum levels. Here, we overcome present experimental and technological challenges to create a three-particle GHZ state entangled in three levels for every particle. The resulting qutrit-entangled states are able to carry more information than entangled states of qubits. Our method, inspired by the computer algorithm Melvin, relies on a new multi-port that coherently manipulates several photons simultaneously in higher dimensions. The realization required us to develop a new high-brightness four-photon source entangled in orbital angular momentum. Our results allow qualitatively new refutations of local-realistic world views. We also expect that they will open up pathways for a further boost to quantum technologies.

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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 authors upon reasonable request.

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## Acknowledgements

We thank J. Lawrence, M. Huber, C. Brukner, A. Hochrainer, R. Fickler, T. Scheidl, F. Steinlechner and X. Gu for fruitful discussions. This work was supported by the Austrian Academy of Sciences (ÖAW), by the European Research Council (SIQS grant no. 600645 EU-FP7-ICT) and the Austrian Science Fund (FWF) with SFB F40 (FOQUS) and FWF project CoQuS no. W1210-N16. M.M. acknowledges support from the European Commission through a Marie Curie fellowship (OAMGHZ) and the joint Czech–Austrian project MultiQUEST (FWF I3053-N27), and the QuantERA ERA-NET Co-fund (FWF I3553-N36).

## Author information

Authors

### Contributions

The computer algorithm Melvin inspired an initial practical solution for the experiment. M.E. and M.M. performed the experiment. All authors analysed the data, discussed the results and wrote the manuscript. A.Z. initiated the research and supervised the project.

### Corresponding authors

Correspondence to Manuel Erhard or Anton Zeilinger.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary notes and figures.

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Reprints and Permissions

Erhard, M., Malik, M., Krenn, M. et al. Experimental Greenberger–Horne–Zeilinger entanglement beyond qubits. Nature Photon 12, 759–764 (2018). https://doi.org/10.1038/s41566-018-0257-6

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