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# A solid-state source of strongly entangled photon pairs with high brightness and indistinguishability

## Abstract

The generation of high-quality entangled photon pairs has been a long-sought goal in modern quantum communication and computation. So far, the most widely used entangled photon pairs have been generated from spontaneous parametric down-conversion (SPDC), a process that is intrinsically probabilistic and thus relegated to a regime of low rates of pair generation. In contrast, semiconductor quantum dots can generate triggered entangled photon pairs through a cascaded radiative decay process and do not suffer from any fundamental trade-off between source brightness and multi-pair generation. However, a source featuring simultaneously high photon extraction efficiency, high degree of entanglement fidelity and photon indistinguishability has been lacking. Here, we present an entangled photon pair source with high brightness and indistinguishability by deterministically embedding GaAs quantum dots in broadband photonic nanostructures that enable Purcell-enhanced emission. Our source produces entangled photon pairs with a pair collection probability of up to 0.65(4) (single-photon extraction efficiency of 0.85(3)), entanglement fidelity of 0.88(2), and indistinguishabilities of 0.901(3) and 0.903(3) (brackets indicate uncertainty on last digit). This immediately creates opportunities for advancing quantum photonic 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 author upon reasonable request.

Journal peer review information: Nature Nanotechnology thanks Weibo Gao, Alastair Sinclair 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.

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## Acknowledgements

We acknowledge R. Trotta, X. Yuan, H. Huang, M. Reindl, D. Huber and Y. Huo for discussions. We are grateful for financial support from the National Key R&D Program of China (2016YFA0301300, 2018YFA0306100), the National Natural Science Foundations of China (91750207, 11674402, 11761141015, 11761131001, 11874437, 11704424), Guangzhou Science and Technology project (201805010004), the Natural Science Foundations of Guangdong (2018B030311027, 2017A030310004, 2016A030310216, 2016A030312012), the national supercomputer center in Guangzhou, the Austrian Science Fund (FWF): P29603, and the LIT Secure and Correct Systems Lab funded by the State of Upper Austria.

## Author information

R.B.S., J. Li and X.W. conceived the nanostructure and its fabrication strategy. J. Liu proposed the entanglement generation and designed the experiments. R.S and K.S. contributed to the structure simulations. S.F.C.d.S. and Y.Y. grew the QD wafers. R.S., B.Y., J. Liu and J. Li fabricated the devices. Y.W., RS., B.Y. and J. Liu characterized the devices. J.I.-S. performed the indistinguishability calculation. J. Liu, Y.W. and R.S. analysed the data. J. Liu wrote the manuscript with inputs from all authors. J. Liu, A.R. and X.W. supervised the project.

### Competing interests

The authors declare no competing interests.

Correspondence to Armando Rastelli or Juntao Li or Xuehua Wang.

## Supplementary information

1. ### Supplementary Information

Supplementary text and Supplementary Figures 1–9

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• #### DOI

https://doi.org/10.1038/s41565-019-0435-9