EACN Workshop 2022 6G Quantum Communication Networks (Caspar Hopfmann)

In recent years quantum networks have attracted enormous interest due to their enticing promises such as distributed quantum computing [1], ultra-precise remote synchronization [2] and distributed quantum sensing combined with inherent physically secure communication schemes [3]. While these perspective use-cases and applications have been analyzed from a theoretical view point, the experimental realization of such systems remains a major challenge. In order to tackle this challenge and realize first practical quantum networks a keen understanding of the current state-of-the-art, the required components as well as their limitations is fundamental. This essential background knowledge will inform current and future research foci of both theory and experiment and enable identification of possible alternatives to existing quantum network paradigms. An example for such an alternative approach are photon graph states – and in particular cluster states – which open up new possibilities for the realization of robust multipartite entanglement distribution in future quantum networks. Recent experimental advances on photonic graph [4] and cluster-states [5], high throughput entangled photon pair sources [6], memory-enhanced entanglement distribution [7] and entanglement swapping [8] bring the goal of practical realization of scalable quantum networks ever closer to reality and promise exciting new perspectives.
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network of clocks," Nature Physics, vol. 10, p. 582–587, 2014.
[3] Y.-A. Chen, Q. Zhang, T.-Y. Chen, W.-Q. Cai, S.-K. Liao, J. Zhang, K. Chen, J. Yin, J.-G. Ren, Z. Chen,
S.-L. Han, Q. Yu, K. Liang, F. Zhou, X. Yuan, M.-S. Zhao, T.-Y. Wang, X. Jiang, L. Zhang, W.-Y. Liu,
Y. Li, Q. Shen, Y. Cao, C.-Y. Lu, R. Shu, J.-Y. Wang, L. Li, N.-L. Liu, F. Xu, X.-B. Wang, C.-Z. Peng and
J.-W. Pan, "An integrated space-to-ground quantum communication network over 4,600 kilometres,"
Nature, vol. 589, p. 214–219, 2021.
[4] X.-L. Wang, Y.-H. Luo, H.-L. Huang, M.-C. Chen, Z.-E. Su, C. Liu, C. Chen, W. Li, Y.-Q. Fang, X.
Jiang, J. Zhang, L. Li, N.-L. Liu, C.-Y. Lu and J.-W. Pan, "18-Qubit Entanglement with Six Photons’
Three Degrees of Freedom," Phys. Rev. Lett., vol. 120, no. 26, p. 260502, 6 2018.
[5] I. Schwartz, D. Cogan, E. R. Schmidgall, Y. Don, L. Gantz, O. Kenneth, N. H. Lindner and D. Gershoni, "Deterministic generation of a cluster state of entangled photons," Science, vol. 354, p. 434–437, 2016.
[6] C. Hopfmann, W. Nie, N. L. Sharma, C. Weigelt, F. Ding and O. G. Schmidt, "Maximally entangled
and gigahertz-clocked on-demand photon pair source," Phys. Rev. B, vol. 103, no. 7, p. 075413, 2
2021.
[7] M. K. Bhaskar, R. Riedinger, B. Machielse, D. S. Levonian, C. T. Nguyen, E. N. Knall, H. Park, D.
Englund, M. Lončar, D. D. Sukachev and M. D. Lukin, "Experimental demonstration of memoryenhanced quantum communication," Nature, vol. 580, p. 60–64, 2020.
[8] M. Zopf, R. Keil, Y. Chen, J. Yang, D. Chen, F. Ding and O. G. Schmidt, "Entanglement Swapping
with Semiconductor-Generated Photons Violates Bell’s Inequality," Phys. Rev. Lett., vol. 123, no.
16, p. 160502, 10 2019.

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