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# A quantum network node with crossed optical fibre cavities

## Abstract

Quantum networks provide unique possibilities for resolving open questions on entanglement1 and promise innovative applications ranging from secure communication to scalable computation2. Although two quantum nodes coupled by a single channel are adequate for basic quantum communication tasks between two parties3, fully functional large-scale quantum networks require a web-like architecture with multiply connected nodes4. Efficient interfaces between network nodes and channels can be implemented with optical cavities5. Using two optical fibre cavities coupled to one atom, we here realize a quantum network node that connects to two quantum channels, one provided by each cavity. It functions as a passive, heralded and high-fidelity quantum memory that requires neither amplitude- and phase-critical control fields6,7,8 nor error-prone feedback loops9. Our node is robust, fits naturally into larger fibre-based networks and has prospects for extensions including qubit-controlled quantum switches10,11, routers12,13 and repeaters14,15.

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## Data availability

Source data for Figs. 1, 3 and 4 are provided with the paper. The data that support the findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

We thank S. Ritter and M. Uphoff for contributions during an early stage of this work and T. Urban for contributions to the design and fabrication of the experimental chamber. This work was supported by the Bundesministerium für Bildung und Forschung via the Verbund Q.Link.X (grant no. 16KIS0870), the Deutsche Forschungsgemeinschaft under Germany’s Excellence Strategy (EXC-2111, 390814868) and the European Union’s Horizon 2020 research and innovation programme via the project Quantum Internet Alliance (GA no. 820445). J.D.C. acknowledges support from the Alexander von Humboldt Foundation.

## Author information

Authors

### Contributions

All authors contributed to the experiment, analysis of the results and writing of the manuscript.

### Corresponding author

Correspondence to Manuel Brekenfeld.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Physics thanks Lijun Ma and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Supplementary information

### Supplementary Information

Supplementary text and Figs. 1–4.

## Source data

### Source Data Fig. 1

Plotted data Fig. 1.

### Source Data Fig. 3

Plotted data Fig. 3.

### Source Data Fig. 4

Plotted data Fig. 4.

## Rights and permissions

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Brekenfeld, M., Niemietz, D., Christesen, J.D. et al. A quantum network node with crossed optical fibre cavities. Nat. Phys. 16, 647–651 (2020). https://doi.org/10.1038/s41567-020-0855-3

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